config: allow vendors to customize the version info

It's common for vendors (such as Linux distros) to want to customize the
User-Agent string and version information for software they provide.
This is valuable for vendors, but it also allows troubleshooting
problems with custom patches that may have been applied by the vendor.
Allow vendors to set a VENDOR makefile flag to set the vendor field for
the User-Agent and version information.

Makefile

@@ -22,6 +22,9 @@ GO_TEST_EXTRA_ARGS =
 # the config.GitCommit variable is always set via this variable, and
 # DWARF-stripping is enabled unless DWARF=YesPlease.
 BUILTIN_LD_FLAGS =
+ifneq ("$(VENDOR)","")
+BUILTIN_LD_FLAGS += -X github.com/git-lfs/git-lfs/config.Vendor=$(VENDOR)
+endif
 BUILTIN_LD_FLAGS += -X github.com/git-lfs/git-lfs/config.GitCommit=$(GIT_LFS_SHA)
 ifneq ("$(DWARF)","YesPlease")
 BUILTIN_LD_FLAGS += -s

config/version.go

@@ -9,6 +9,7 @@ import (
 var (
 	GitCommit   string
 	VersionDesc string
+	Vendor      string
 )
 
 const (
@@ -20,8 +21,12 @@ func init() {
 	if len(GitCommit) > 0 {
 		gitCommit = "; git " + GitCommit
 	}
-	VersionDesc = fmt.Sprintf("git-lfs/%s (GitHub; %s %s; go %s%s)",
+	if len(Vendor) == 0 {
+		Vendor = "GitHub"
+	}
+	VersionDesc = fmt.Sprintf("git-lfs/%s (%s; %s %s; go %s%s)",
 		Version,
+		Vendor,
 		runtime.GOOS,
 		runtime.GOARCH,
 		strings.Replace(runtime.Version(), "go", "", 1),