merge: tool.check = prompt will force an interactive merge check

tool.check = prompt can be used when the exit status of a merge
tool is unreliable but an explicit user signoff on the merge result is
acceptable.

doc/hgrc.5.txt

@@ -470,6 +470,8 b' Supported arguments:'
     Ask whether merge was successful when the merged file shows no changes.
   ``conflicts``
     Check whether there are conflicts even though the tool reported success.
+  ``prompt``
+    Always prompt for merge success, regardless of success reported by tool.
 
 ``checkchanged``
   True is equivalent to ``check = changed``.

mercurial/filemerge.py

@@ -231,8 +231,15 b' def filemerge(repo, mynode, orig, fcd, f'
         if re.match("^(<<<<<<< .*|=======|>>>>>>> .*)$", fcd.data()):
             r = 1
 
-    if not r and (_toolbool(ui, tool, "checkchanged") or
-                  'changed' in _toollist(ui, tool, "check")):
+    checked = False
+    if 'prompt' in _toollist(ui, tool, "check"):
+        checked = True
+        if ui.promptchoice(_("was merge of '%s' successful (yn)?") % fd,
+                           (_("&Yes"), _("&No")), 1):
+            r = 1
+
+    if not r and not checked and (_toolbool(ui, tool, "checkchanged") or
+                                  'changed' in _toollist(ui, tool, "check")):
         if filecmp.cmp(repo.wjoin(fd), back):
             if ui.promptchoice(_(" output file %s appears unchanged\n"
                                  "was merge successful (yn)?") % fd,