merge-tools: add a `premerge=keep-merge3` config option

This value leaves premerge markers that includes the merge base too. This is a
the same as what `internal:merge3` would do.

mercurial/filemerge.py

@@ -178,7 +178,7 def _premerge(repo, toolconf, files, lab
 
     ui = repo.ui
 
-    validkeep = ['keep']
+    validkeep = ['keep', 'keep-merge3']
 
     # do we attempt to simplemerge first?
     try:
@@ -192,6 +192,11 def _premerge(repo, toolconf, files, lab
                                     (tool, premerge, _valid))
 
     if premerge:
+        if premerge == 'keep-merge3':
+            if not labels:
+                labels = _defaultconflictlabels
+            if len(labels) < 3:
+                labels.append('base')
         r = simplemerge.simplemerge(ui, a, b, c, quiet=True, label=labels)
         if not r:
             ui.debug(" premerge successful\n")

mercurial/help/config.txt

@@ -954,8 +954,10 Supported arguments:
 
 ``premerge``
   Attempt to run internal non-interactive 3-way merge tool before
-  launching external tool.  Options are ``true``, ``false``, or ``keep``
-  to leave markers in the file if the premerge fails.
+  launching external tool.  Options are ``true``, ``false``, ``keep`` or
+  ``keep-merge3``. The ``keep`` option will leave markers in the file if the
+  premerge fails. The ``keep-merge3`` will do the same but include information
+  about the base of the merge in the marker (see internal:merge3).
   Default: True
 
 ``binary``

tests/test-merge-tools.t

@@ -741,6 +741,44 premerge=keep keeps conflict markers in:
   # hg stat
   M f
 
+premerge=keep-merge3 keeps conflict markers with base content:
+
+  $ beforemerge
+  [merge-tools]
+  false.whatever=
+  true.priority=1
+  true.executable=cat
+  # hg update -C 1
+  $ hg merge -r 4 --config merge-tools.true.premerge=keep-merge3
+  merging f
+  <<<<<<< local: ef83787e2614  - test: revision 1
+  revision 1
+  space
+  ||||||| base
+  revision 0
+  space
+  =======
+  revision 4
+  >>>>>>> other: 81448d39c9a0 - test: revision 4
+  revision 0
+  space
+  revision 4
+  0 files updated, 1 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  $ aftermerge
+  # cat f
+  <<<<<<< local: ef83787e2614  - test: revision 1
+  revision 1
+  space
+  ||||||| base
+  revision 0
+  space
+  =======
+  revision 4
+  >>>>>>> other: 81448d39c9a0 - test: revision 4
+  # hg stat
+  M f
+
 
 Tool execution