[PATCH] merge.graft: add option to keep second parent

[Andrew Halberstadt]

# HG changeset patch
# User Andrew Halberstadt <ahalberstadt at mozilla.com>
# Date 1449201719 18000
#      Thu Dec 03 23:01:59 2015 -0500
# Node ID 212f93c2367203cadfd6103f444899929b8643ae
# Parent  71aa5a26162d6e4a165b68f07b331e3e2eedc117
merge.graft: add option to keep second parent

Currently merge.graft re-writes the dirstate so only a single
parent is kept. For some cases, like evolving a merge commit,
this behaviour is not desired. More specifically, this is
needed to fix issue4389.

diff --git a/mercurial/merge.py b/mercurial/merge.py
--- a/mercurial/merge.py
+++ b/mercurial/merge.py
@@ -1484,41 +1484,47 @@ def update(repo, node, branchmerge, forc
             repo.dirstate.endparentchange()
     finally:
         wlock.release()
 
     if not partial:
         repo.hook('update', parent1=xp1, parent2=xp2, error=stats[3])
     return stats
 
-def graft(repo, ctx, pctx, labels):
+def graft(repo, ctx, pctx, labels, keepparent=False):
     """Do a graft-like merge.
 
     This is a merge where the merge ancestor is chosen such that one
     or more changesets are grafted onto the current changeset. In
     addition to the merge, this fixes up the dirstate to include only
-    a single parent and tries to duplicate any renames/copies
-    appropriately.
+    a single parent (if keepparent is False) and tries to duplicate any
+    renames/copies appropriately.
 
     ctx - changeset to rebase
     pctx - merge base, usually ctx.p1()
     labels - merge labels eg ['local', 'graft']
+    keepparent - keep second parent if any
 
     """
     # If we're grafting a descendant onto an ancestor, be sure to pass
     # mergeancestor=True to update. This does two things: 1) allows the merge if
     # the destination is the same as the parent of the ctx (so we can use graft
     # to copy commits), and 2) informs update that the incoming changes are
     # newer than the destination so it doesn't prompt about "remote changed foo
     # which local deleted".
     mergeancestor = repo.changelog.isancestor(repo['.'].node(), ctx.node())
 
     stats = update(repo, ctx.node(), True, True, False, pctx.node(),
                    mergeancestor=mergeancestor, labels=labels)
 
-    # drop the second merge parent
+    pother = nullid
+    parents = ctx.parents()
+    if keepparent and len(parents) == 2 and pctx in parents:
+        parents.remove(pctx)
+        pother = parents[0].node()
+
     repo.dirstate.beginparentchange()
-    repo.setparents(repo['.'].node(), nullid)
+    repo.setparents(repo['.'].node(), pother)
     repo.dirstate.write(repo.currenttransaction())
     # fix up dirstate for copies and renames
     copies.duplicatecopies(repo, ctx.rev(), pctx.rev())
     repo.dirstate.endparentchange()
     return stats