vendor: update go-git to master and add push optimizations

Compared to the v4 branch we were using before, master branch seems to
fix a few bugs and bring the time for pushing the KBFS repo from hours
to about 25 minutes.  With the following optimizations, I got that
time down to less than 3 minutes.

* src-d/go-git#564
* src-d/go-git#565

I suspect there are still lots more optimizations that can be done.  I
don't think there are many users of Push in this project.

Author: strib

kbfsgit/runner.go

@@ -351,7 +351,7 @@ func (r *runner) handleFetchBatch(ctx context.Context, args [][]string) (
 
 	remote, err := repo.CreateRemote(&gogitcfg.RemoteConfig{
 		Name: localRepoRemoteName,
-		URL:  r.gitDir,
+		URLs: []string{r.gitDir},
 	})
 
 	for _, fetch := range args {
@@ -439,7 +439,7 @@ func (r *runner) handlePushBatch(ctx context.Context, args [][]string) (
 
 	remote, err := repo.CreateRemote(&gogitcfg.RemoteConfig{
 		Name: localRepoRemoteName,
-		URL:  r.gitDir,
+		URLs: []string{r.gitDir},
 	})
 
 	results := make(map[string]error, len(args))