upload: improve tip for fixing upload remote

Instead of assuming the repo client is tracking the "master" branch
of the manifest repo, use the existing info we have to display the
right info to the user.

Bug: https://crbug.com/gerrit/13339
Change-Id: I8b265f4b2e075fdc41909b1f3dff9aee87384353
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/287279
Reviewed-by: Michael Mortensen <mmortensen@google.com>
Tested-by: Mike Frysinger <vapier@google.com>

subcmds/upload.py

@@ -554,10 +554,10 @@ Gerrit Code Review:  https://www.gerritcodereview.com/
           avail = [up_branch]
         else:
           avail = None
-          print('ERROR: Current branch (%s) not uploadable. '
-                'You may be able to type '
-                '"git branch --set-upstream-to m/master" to fix '
-                'your branch.' % str(cbr),
+          print('repo: error: Unable to upload branch "%s". '
+                'You might be able to fix the branch by running:\n'
+                '  git branch --set-upstream-to m/%s' %
+                (str(cbr), self.manifest.branch),
                 file=sys.stderr)
       else:
         avail = project.GetUploadableBranches(branch)