merge: allow user to halt merge on merge-tool failures

Depends on D932.

Call the new _onfilemergefailure function when a merge tool reports failure
via a return code.

Differential Revision: https://phab.mercurial-scm.org/D951

mercurial/filemerge.py

@@ -739,6 +739,7 b' def _filemerge(premerge, repo, wctx, myn'
         if r:
             if onfailure:
                 ui.warn(onfailure % fd)
+            _onfilemergefailure(ui)
 
         return True, r, deleted
     finally:

tests/test-merge-halt.t

@@ -44,8 +44,7 b' Testing on-failure=halt'
   merging a
   merging b
   merging a failed!
-  merging b failed!
-  unresolved conflicts (see hg resolve, then hg rebase --continue)
+  merge halted after failed merge (see hg resolve)
   [1]
 
   $ hg resolve --list
@@ -65,8 +64,10 b' Testing on-failure=prompt'
   merging a
   merging b
   merging a failed!
+  continue merge operation (yn)? y
   merging b failed!
-  unresolved conflicts (see hg resolve, then hg rebase --continue)
+  continue merge operation (yn)? n
+  merge halted after failed merge (see hg resolve)
   [1]
 
   $ hg resolve --list