SAW WIRE AND METHOD OF MANUFACTURING SAW WIRE TECHNICAL FIELD The present invention relates to a saw wire and a manufacturing method thereof, particularly to a fixed-abrasive grain saw wire and a manufacturing method thereof. BACKGROUND ART As a method of slicing a semiconductor ingot such as silicon, SiC or the like, there has been used ID blade cutting capable of performing slicing with a high efficiency and precision. According to such ID blade cutting, there has been an attempt to enlarge the diameter of an ID blade to match an enlarged silicon ingot, and an attempt to thin the ID blade to reduce a cutting loss. However, the ID blade cannot be thinned any further when the diameter of the silicon ingot is greater than 12 inch (300mm). In this sense, there has been a limitation on a reduction of a cutting loss of an expensive work material. Here, as an alternative to the ID blade cutting method for cutting a semiconductor ingot, there has been performed a wire cutting method employing loose abrasive grains of WA (white alumina), GC (green silicon carbide) or the like mixed in an oil. Further, in order to slice a single-crystal ingot such as sapphire, SiC or the like, there has been performed a cutting method employing loose abrasive grains of WA (white alumina), GC (green silicon carbide), diamond or the like mixed in an oil However, this type of cutting method has exhibited not only a low cutting speed, but also an unsatisfactory precision. Further, when performing this type of cutting method, a work material may be smeared with an oil, thus leading to environmental pollution. On the other hand, there has been performed a cutting method employing a fixed-abrasive grain saw wire. According to this method, a saw wire with super abrasive grains adhered to a steel wire or the like is stretched across guiding rolls, and an appropriate amount of load is applied to a semiconductor ingot or the like serving as a work material. In this way, the work material is allowed to be in contact with the saw wire, thereby allowing a work material with a large diameter to be cut by a thin saw wire and reducing a cutting loss thereof. Further, a water-based grinding fluid can be used when performing this wire cutting method. As conventional methods for manufacturing a diamond saw wire employing diamond abrasive grains as super grains, there have been proposed a method in which coarse diamond abrasive grains are first adhered via Ni electrodeposition, and fine diamond abrasive grains are adhered later also via Ni electrodeposition (Patent document 1: Japanese Examined Patent Application Publication No.Hei 4-4105), and a method in which a wire is helically wound for several times as well as held by a wire suspender, and Ni plating is statically performed by inserting such wire into a abrasive grain tank (Patent document 2: Japanese Unexamined Patent Application Publication No. Hei 7-227766). As for fixed-abrasive grain saw wires obtained without performing Ni electrodeposition, there have been known a fixed-resin saw wire employing a resin (Patent document 3: Japanese Unexamined Patent Application Publication No.2006-7387), and a saw wire with abrasive grains adhered thereto via a brazing material and a solder (Patent document 4: Japanese Patent No.4008660). However, a sufficient cutting performance cannot be achieved with the fixed-resin saw wire, since the resin has an insufficient adhesion force for adhering the diamond abrasive grains. Further, with regard to the Ni electrodeposited saw wire, there needs to be secured on a diamond abrasive grain bed abrasive grains of an amount far larger than an amount of the abrasive grains actually adhered to a wire, and an electrodeposition speed is slow. Therefore, there must be held a large amount of expensive diamond abrasive grains for a long time. Thus, despite a relatively high cutting performance thereof, the Ni electrodeposited saw wire has to be manufactured at a high cost. In contrast to those manufacturing methods, Patent document 4 discloses a manufacturing method using a brazing material and a solder, such manufacturing method being relatively low-cost and capable of forming a saw wire with a relatively high cutting performance. An example 3 and example 4 of the Patent document 4 disclose a manufacturing method and an evaluation of a diamond-coated obtained by allowing Cu-plated diamond abrasive grains to be adhered to the Inconel 718 wire using an Sn-Ag-Cu-based solder. PRIOR ART LITERATURES Patent Literatures Patent document 1: Japanese Examined Patent Application Publication No.Hei 4-4105 Patent document 2: Patent document 2: Japanese Unexamined Patent Application Publication No. Hei 7-227766 Patent document 3: Patent document 3: Japanese Unexamined Patent Application Publication No.2006-7387 Patent document 4: Patent document 4: Japanese Patent No.4008660 DISCLOSURE OF THE INVENTION Problem to be solved by the Invention However, a sufficient holding force for holding the abrasive grains cannot be achieved event with the saw wire disclosed in Patent document 4, thereby making it difficult to improve the cutting performance of the corresponding saw wire. In view of the aforementioned problems, it is an object of the present invention to provide a saw wire with a superior cutting performance, with abrasive grains adhered to a metal wire and with fewer abrasive grains dropping off, and a manufacturing method thereof. Mean for Solving the Problem The invention according to an aspect 1 is a fixed-abrasive grain saw wire with abrasive grains adhered to a metal wire through an adhesion portion, in which the adhesion portion includes a Zn-based or Sn-based low-melting-point metal and a high-melting-point metal with a melting point higher than that of the low-melting-point metal. [0012] The invention according to an aspect 2 is the one set forth in the aspect 1, in which the high-melting-point metal is contained in a high-melting-point metal layer formed on at least one of areas including peripheries of the abrasive grains, a surface of the saw wire and a surface of the metal wire. The invention according to an aspect 3 is the one set forth in the aspect 1, in which the abrasive grains are coated with a Ni coating layer, the low-melting-point metal is an Sn-based solder containing 0.5 to 5.0% by mass of Ag, and the high-melting-point metal is an intermetallic compound containing Sn. The invention according to an aspect 4 is the one set forth in the aspect 3, in which the Sn-based solder containing 0.5 to 5.0% by mass of Ag has a structure in which there are dispersed AgjSn-based intermetallic compounds that are at least one of a plate type having a thickness of 1 to 2um and a cord type having a diameter of 1 to 2um. The invention according to an aspect 5 is the one set forth in the aspect 3 or the aspect 4, in which the Sn-based solder containing 0.5 to 5.0% by mass of Ag further contains 0.01 to 0.5% by mass of Fe and 0.01 to 0.5% by mass of Ni. The invention according to an aspect 6 is the one set forth in the aspect 3, in which the abrasive grains comprise a multi-layered coating layer having a lower layer of Ti or Cr and an upper layer of Ni. The invention according to an aspect 7 is the one set forth in the aspect 3 or the aspect 6, in which the intermetallic compound containing Sn exists at least around the abrasive grains. The invention according to an aspect 8 is the one set forth in the aspect 2 or the aspect 6, m which an intermetallic compound containing Sn and Ni exists as an intermetallic compound layer around the abrasive grains. The invention according to an aspect 9 is the one set forth in the aspect 2 or the aspect 6, in which an intermetallic compound containing Sn and Ni includes one or more than one of Ni3SN4, Ni3Sn2 and Sn(i.x.y)NixCuy (here, 0.1 < x <0.7, 0.01