use tag if version name is unknown

If we don't find the version name neither in the AndroidManifest.xml nor
in the build.grade file, we fallback to the tag the user specified to
search for.

That way we have a better version name than 'Unknown'.

fdroidserver/checkupdates.py

@@ -388,6 +388,8 @@ def checkupdates_app(app, first=True):
     if mode.startswith('Tags'):
         pattern = mode[5:] if len(mode) > 4 else None
         (version, vercode, tag) = check_tags(app, pattern)
+        if version == 'Unknown':
+            version = tag
         msg = vercode
     elif mode == 'RepoManifest':
         (version, vercode) = check_repomanifest(app)