Check branch existence before merge

We now support merges happenning between stage and release. But the code
for this does not currently check to see if the remote branch exists
before attempting to fetch and merge.

This commit modifies the git pusher to check if the remote branch exists
before merging.

Signed-off-by: Adolfo García Veytia (Puerco) <adolfo.garcia@uservers.net>

pkg/release/push_git_objects.go

@@ -248,6 +248,19 @@ func (gp *GitObjectPusher) mergeRemoteIfRequired(branch string) error {
 		return errors.Wrap(err, "fetch remote")
 	}
 
+	branchExists, err := gp.repo.HasRemoteBranch(branch)
+	if err != nil {
+		return errors.Wrapf(
+			err, "checking if branch %s exists in repo remote", branch,
+		)
+	}
+	if !branchExists {
+		logrus.Infof(
+			"Git repository does not have remote branch %s, not attempting merge", branch,
+		)
+		return nil
+	}
+
 	logrus.Infof("Merging %s branch", branch)
 	if err := gp.repo.Merge(branch); err != nil {
 		return errors.Wrapf(