merge: add an internal:merge3 tool This variant gives access to a feature already present in ``internal:merge``: displaying merge base content. In the basic merge (calling ``hg merge``) case, including more context to the merge markers is an interesting addition. But this extra information is the only viable option in case conflict from grafting (, rebase, etc?). When grafting ``source`` on ``destination``, the parent of ``source`` is used as the ``base``. When all three changesets add content in the same location, the marker for ``source`` will contains both ``base`` and ``source`` content. Without the content of base exposed, there is no way for the user to discriminate content coming from ``base`` and content commit from ``source``. Practical example (all addition are in the same place): * ``destination`` adds ``Dest-Content`` * ``base`` adds ``Base-Content`` * ``source`` adds ``Src-Content`` Grafting ``source`` on ``destination`` will produce the following conflict: <<<<<<< destination Dest-Content ======= Base-Content Src-Content >>>>>>> source This that case there is no way to distinct ``base`` from ``source``. As a result content from ``base`` are likely to slip in the resolution result. However, adding the base make the situation very clear: <<<<<<< destination Dest-Content ||||||| base Base-Content ======= base Base-Content Src-Content >>>>>>> source Once the base is added, the addition from the grafted changeset is made clear. User can compare the content from ``base`` and ``source`` to make an enlightened decision during merge resolution.

--- a/mercurial/filemerge.py	Tue Aug 05 15:09:54 2014 -0700
+++ b/mercurial/filemerge.py	Tue Aug 05 14:58:45 2014 -0700
@@ -223,6 +223,21 @@
         return True, r
     return False, 0
 
+@internaltool('merge3', True,
+              _("merging %s incomplete! "
+                "(edit conflicts, then use 'hg resolve --mark')\n"))
+def _imerge3(repo, mynode, orig, fcd, fco, fca, toolconf, files, labels=None):
+    """
+    Uses the internal non-interactive simple merge algorithm for merging
+    files. It will fail if there are any conflicts and leave markers in
+    the partially merged file. Marker will have three sections, one from each
+    side of the merge and one for the base content."""
+    if not labels:
+        labels = _defaultconflictlabels
+    if len(labels) < 3:
+        labels.append('base')
+    return _imerge(repo, mynode, orig, fcd, fco, fca, toolconf, files, labels)
+
 @internaltool('tagmerge', True,
               _("automatic tag merging of %s failed! "
                 "(use 'hg resolve --tool internal:merge' or another merge "