git: fix change testing when version is HEAD (ansible#3025)

When `version` is not specified, it defaults to "HEAD".  "HEAD" is not a
remote tag, and it's not listed in the output of get_branches(), so we'd
keep repo_updated at the default value (None) and then return early with
changed=True in --check mode, even when before == after.

Fixes ansible#3024.

lib/ansible/modules/source_control/git.py

@@ -819,6 +819,8 @@ def main():
             if local_mods:
                 module.exit_json(changed=True, before=before, after=remote_head,
                     msg="Local modifications exist")
+            elif version == 'HEAD':
+                repo_updated = False
             elif is_remote_tag(git_path, module, dest, repo, version):
                 # if the remote is a tag and we have the tag locally, exit early
                 if version in get_tags(git_path, module, dest):