push: Make --debian-tag='' match the documentation

An empty Debian tag indicates "don't care" about the packaging branch.
Don't fail in that case but rather push out up to the branch tip:

  Currently we'd fail like

  $ gbp push --debian-tag='' guido
  Traceback (most recent call last):
    File "/usr/bin/gbp", line 149, in <module>
      sys.exit(supercommand())
    File "/usr/bin/gbp", line 145, in supercommand
      return module.main(args)
    File "/usr/lib/python3/dist-packages/gbp/scripts/push.py", line 153, in main
      to_push['refs'].append((ref, get_push_src(repo, ref, dtag)))
  UnboundLocalError: local variable 'dtag' referenced before assignment

this make it simple to push the current development work via

  gbp  push --debian-tag='' <remote>

1 files changed, 8 insertions, 2 deletions

diff --git a/gbp/scripts/push.py b/gbp/scripts/push.py
index 8233ace8..d6876e26 100755
--- a/gbp/scripts/push.py
+++ b/gbp/scripts/push.py
@@ -133,6 +133,8 @@ def main(argv):
     try:
         source = DebianSource(repo.path)
         branch = repo.branch
+        dtag = None
+
         if not options.ignore_branch:
             if branch != options.debian_branch:
                 gbp.log.err("You are not on branch '%s' but %s" %
@@ -148,9 +150,13 @@ def main(argv):
             if repo.has_tag(dtag):
                 to_push['tags'].append(dtag)
 
-        if source.is_releasable() and branch:
+        if branch:
             ref = 'refs/heads/%s' % branch
-            to_push['refs'].append((ref, get_push_src(repo, ref, dtag)))
+            if source.is_releasable() and dtag:
+                to_push['refs'].append((ref, get_push_src(repo, ref, dtag)))
+            elif not dtag:
+                # --debian-tag='': Push branch up to tip
+                to_push['refs'].append((ref, get_push_src(repo, ref, ref)))
 
         if not source.is_native():
             if options.upstream_tag != '':