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

--- a/tests/test-pathconflicts-basic.t	Thu Jan 04 21:37:03 2018 -0800
+++ b/tests/test-pathconflicts-basic.t	Mon Jan 08 19:41:47 2018 +0530
@@ -39,7 +39,7 @@
   moving a to a~853701544ac3
   getting a/b
   1 files updated, 0 files merged, 0 files removed, 1 files unresolved
-  use 'hg resolve' to retry unresolved file merges or 'hg update -C .' to abandon
+  use 'hg resolve' to retry unresolved file merges or 'hg merge --abort' to abandon
   [1]
   $ hg update --clean .
   1 files updated, 0 files merged, 1 files removed, 0 files unresolved