mr: Default to remote tracking branch

We currently assume that the local branch name matches the remote one,
which isn't necessarily the case. Address this by using the remote
tracking name where appropriate.

cmd/mr_create.go

@@ -105,15 +105,25 @@ func runMRCreate(cmd *cobra.Command, args []string) {
 		log.Fatal(err)
 	}
 
+	remoteBranch, err := git.CurrentUpstreamBranch()
+	if remoteBranch == "" {
+		// Fall back to local branch
+		remoteBranch, err = git.CurrentBranch()
+	}
+
+	if err != nil {
+		log.Fatal(err)
+	}
+
 	p, err := lab.FindProject(sourceProjectName)
 	if err != nil {
 		log.Fatal(err)
 	}
-	if _, err := lab.GetCommit(p.ID, branch); err != nil {
+	if _, err := lab.GetCommit(p.ID, remoteBranch); err != nil {
 		err = errors.Wrapf(
 			err,
 			"aborting MR, source branch %s not present on remote %s. did you forget to push?",
-			branch, sourceRemote)
+			remoteBranch, sourceRemote)
 		log.Fatal(err)
 	}
 

cmd/util.go

@@ -69,9 +69,14 @@ func flagConfig(fs *flag.FlagSet) {
 func getCurrentBranchMR(rn string) int {
 	var num int = 0
 
-	currentBranch, err := git.CurrentBranch()
+	currentBranch, err := git.CurrentUpstreamBranch()
+	if currentBranch == "" {
+		// Fall back to local branch
+		currentBranch, err = git.CurrentBranch()
+	}
+
 	if err != nil {
-		log.Fatal(err)
+		return 0
 	}
 
 	mrs, err := lab.MRList(rn, gitlab.ListProjectMergeRequestsOptions{

internal/git/git.go

@@ -131,6 +131,33 @@ func CurrentBranch() (string, error) {
 	return strings.TrimSpace(string(branch)), nil
 }
 
+// CurrentUpstreamBranch returns the upstream of the currently checked out branch
+func CurrentUpstreamBranch() (string, error) {
+	localBranch, err := CurrentBranch()
+	if err != nil {
+		return "", err
+	}
+
+	branch, err := UpstreamBranch(localBranch)
+	if err != nil {
+		return "", err
+	}
+	return branch, nil
+}
+
+// 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()
+	if err != nil {
+		return "", errors.Errorf("No upstream for branch '%s'", branch)
+	}
+	upstreamBranch := strings.SplitN(string(ref), "/", 2)[1]
+	return strings.TrimSpace(upstreamBranch), nil
+}
+
 // PathWithNameSpace returns the owner/repository for the current repo
 // Such as zaquestion/lab
 // Respects GitLab subgroups (https://docs.gitlab.com/ce/user/group/subgroups/)