Fix: Ensure tag and object are pushed together

Fix: Ensure tag and object are pushed together

Address the problem where the tag is not pushed to the mainline
branch therefore causing the tag missing in the git history.

To fix this check commit count between the HEAD and
origin/${GERRIT_BRANCH} before the fetch and merge operation.
This is done to ensure that the tag lands on the target branch.
If the branch has already moved forward from the tagging point,
then a spur commit is created for the tag.

Change-Id: I675c815d4ac39244adf5168c1fbd051f8c626290
Signed-off-by: Anil Belur <abelur@linuxfoundation.org>