git: fix UpstreamBranch

UpstreamBranch is assuming the value returned by `git rev-parse` match
'<remote name>/<branch name>' and split the string on the first / to
retrieve the remote branch name. That only works when the remote name
does not have slashes in its name, otherwise the returned branch name is
invalid and not pointing to a git valid object:

- If the remote is 'origin', the rev-parse returned value would be like
  'origin/branch/name' and UpstreamBranch would return 'branch/name'.

- But if the remote is 'gitlab/origin', the rev-parse returned value
  would be like 'gitlab/origin/branch/name' and UpstreamBranch would
  return 'origin/branch/name'.

`git rev-parse --abbrev-ref @{upstream}` constructs its value by
appending branch.<name>.remote and branch.<name>.merge (and removing
'refs/heads/' from the merge part) according to its man page.

What we really want here is branch.<name>.merge (without 'refs/heads/'),
not @{upstream}. This patch is implementing this.

Signed-off-by: Antoine Tenart <atenart@kernel.org>

internal/git/git.go

@@ -147,15 +147,11 @@ func CurrentUpstreamBranch() (string, error) {
 
 // UpstreamBranch returns the upstream of the specified branch
 func UpstreamBranch(branch string) (string, error) {
-	cmd := New("rev-parse", "--abbrev-ref", branch+"@{upstream}")
-	cmd.Stdout = nil
-	cmd.Stderr = nil
-	ref, err := cmd.Output()
+	upstreamBranch, err := gitconfig.Local("branch." + branch + ".merge")
 	if err != nil {
 		return "", errors.Errorf("No upstream for branch '%s'", branch)
 	}
-	upstreamBranch := strings.SplitN(string(ref), "/", 2)[1]
-	return strings.TrimSpace(upstreamBranch), nil
+	return strings.TrimPrefix(upstreamBranch, "refs/heads/"), nil
 }
 
 // PathWithNameSpace returns the owner/repository for the current repo