merge: add `--abort` flag which can abort the merge 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 etc. 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. .. feature:: A `--abort` flag to merge command to abort the ongoing merge. Differential Revision: https://phab.mercurial-scm.org/D1829

filemerge: only write in-memory backup during premerge This wasn't broken, but should mirror the non-in memory case to save an extra write. Differential Revision: https://phab.mercurial-scm.org/D1807