filemerge: introduce functions to halt merge flow

Depends on D931.

This patch introduces functions and a config option that will allow a user
to halt the merge if there are failures during a file merge. These functions
will be used in the next patch.

Differential Revision: https://phab.mercurial-scm.org/D932

mercurial/configitems.py

@@ -531,6 +531,9 coreconfigitem('merge', 'checkignored',
 coreconfigitem('merge', 'followcopies',
     default=True,
 )
+coreconfigitem('merge', 'on-failure',
+    default='continue',
+)
 coreconfigitem('merge', 'preferancestor',
         default=lambda: ['*'],
 )

mercurial/filemerge.py

@@ -745,6 +745,20 def _filemerge(premerge, repo, wctx, myn
         if not r and back is not None:
             back.remove()
 
+def _haltmerge():
+    msg = _('merge halted after failed merge (see hg resolve)')
+    raise error.InterventionRequired(msg)
+
+def _onfilemergefailure(ui):
+    action = ui.config('merge', 'on-failure')
+    if action == 'prompt':
+        msg = _('continue merge operation (yn)?' '$$ &Yes $$ &No')
+        if ui.promptchoice(msg, 0) == 1:
+            _haltmerge()
+    if action == 'halt':
+        _haltmerge()
+    # default action is 'continue', in which case we neither prompt nor halt
+
 def _check(repo, r, ui, tool, fcd, files):
     fd = fcd.path()
     unused, unused, unused, back = files

mercurial/help/config.txt

@@ -1288,6 +1288,17 This section specifies behavior during m
    different contents. Similar to ``merge.checkignored``, except for files that
    are not ignored. (default: ``abort``)
 
+``on-failure``
+   When set to ``continue`` (the default), the merge process attempts to
+   merge all unresolved files using the merge chosen tool, regardless of
+   whether previous file merge attempts during the process succeeded or not.
+   Setting this to ``prompt`` will prompt after any merge failure continue
+   or halt the merge process. Setting this to ``halt`` will automatically
+   halt the merge process on any merge tool failure. The merge process
+   can be restarted by using the ``resolve`` command. When a merge is
+   halted, the repository is left in a normal ``unresolved`` merge state.
+   (default: ``continue``)
+
 ``merge-patterns``
 ------------------