Pre-calculate the absolute path of git (#6575)

* Pre-caculate the absolute path of git

* Do not repeat string literals which has been defined somewhere

Also make it flexible to accept customized/user-defined value.

modules/git/command.go

@@ -41,7 +41,7 @@ func NewCommand(args ...string) *Command {
 	cargs := make([]string, len(GlobalCommandArgs))
 	copy(cargs, GlobalCommandArgs)
 	return &Command{
-		name: "git",
+		name: GitExecutable,
 		args: append(cargs, args...),
 	}
 }