pkg-merge-updates: Try to fast-forward before rebasing

Explicitly attempt a fast forward merge before rebasing.

Currently we always try to rebase, which already doews a fast-forward
merge when possible. The issue is that after that we incoditionally try
to append a new changelog entry and if we're fast-forwarding we should
**not** to that since the changelog will already have a version equal or
newer than the one we're trying to add.

By explicitly attempting a fast-forward merge and exiting early we avoid
that issue.

This has not been a particular issue so far since the mirroring
pipeline which uses the rebasing feature does not usually trigger it
when a fast-forward is possible and will create all the branches in
their final state.

However in some cases unrelated errors encountered during the process
have left repositories in a inconsistent state and this change allows to
recover cleanly from those situations.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>