Support GitHub Pull Request refspec

This patch introduces support for settings a dep constraint's revision
to a commit that exists in a GitHub pull request. Issue #1583 outlines
how GitHub PRs use a non-standard Git refspec, `+refs/pull/PR_ID`, to
store the commit built by CI systems such as Travis-CI. This patch
includes a new refspec when Git's VCS handler performs a fetch
operation.

The refspec `+refs/pull/*:refs/pull/%x/*` is used where `%x` is an MD5
hash of the constraint's configured remote location. This ensures that
if the remote location is changed, the refspec will point to a new
location locally.

This unblocks build failures occurring in rexray/gocsi#71 and
rexray/gocsi#80.

cc @sbezverk @sdboyer

Author: akutz

gps/vcs_repo.go

@@ -6,6 +6,7 @@ package gps
 
 import (
 	"context"
+	"crypto/md5"
 	"encoding/xml"
 	"fmt"
 	"os"
@@ -103,13 +104,20 @@ func (r *gitRepo) get(ctx context.Context) error {
 }
 
 func (r *gitRepo) fetch(ctx context.Context) error {
+
+	// Hash the remote location to generate the name
+	// of the local refs/pulls namespace.
+	h := md5.New()
+	h.Write([]byte(r.RemoteLocation))
+
 	cmd := commandContext(
 		ctx,
 		"git",
 		"fetch",
 		"--tags",
 		"--prune",
 		r.RemoteLocation,
+		fmt.Sprintf("+refs/pull/*:refs/pull/%x/*", h.Sum(nil)),
 	)
 	cmd.SetDir(r.LocalPath())
 	// Ensure no prompting for PWs