Add what-to-do when push after merge fails

[bram-perl]

The instructions on what to do when a git push after a git merge --no-ff failed were missing/weren't clear.

This PR adds a small section in perlgit.pod on what can be done to preserve the merge.

(Context: @khwilliamson attempted to merge a branch with a merge commit but the merge commit got lost due to a git pull --rebase :-( )