sync: warn if not tracking a branch

Since tracking a branch prevents repo from updating, make sure we
warn people about the situation when using `repo sync`.

Bug: https://crbug.com/gerrit/11045
Change-Id: I966513f510827cc93194f8df176c6745946bd739
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/258892
Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: David Pursehouse <dpursehouse@collab.net>

subcmds/sync.py

@@ -845,6 +845,13 @@ later is required to fix a server side protocol bug.
 
     rp = self.manifest.repoProject
     rp.PreSync()
+    cb = rp.CurrentBranch
+    if cb:
+      base = rp.GetBranch(cb).merge
+      if not base or not base.startswith('refs/heads/'):
+        print('warning: repo is not tracking a remote branch, so it will not '
+              'receive updates; run `repo init --repo-branch=stable` to fix.',
+              file=sys.stderr)
 
     mp = self.manifest.manifestProject
     mp.PreSync()