Currently we don't have a good functionality to abort the merge and tell user to
do hg update -C . which can leads to different results if user missed the '.'
and moreover does not align with other abort functionalities like rebase, shelve
This patch adds hg merge --abort which will abort the ongoing merge and take
us back to the chagneset where we started from. Works in both cases when merge
resulted in conflicts and when there were no conflicts.
A `--abort` flag to merge command to abort the ongoing merge.