--- a/mercurial/localrepo.py	Sun Aug 30 13:58:11 2015 -0700
+++ b/mercurial/localrepo.py	Sun Aug 30 14:03:32 2015 -0700
@@ -354,6 +354,10 @@
         manifestcachesize = self.ui.configint('format', 'manifestcachesize')
         if manifestcachesize is not None:
             self.svfs.options['manifestcachesize'] = manifestcachesize
+        # experimental config: format.aggressivemergedeltas
+        aggressivemergedeltas = self.ui.configbool('format',
+            'aggressivemergedeltas', False)
+        self.svfs.options['aggressivemergedeltas'] = aggressivemergedeltas
 
     def _writerequirements(self):
         scmutil.writerequires(self.vfs, self.requirements)

--- a/mercurial/revlog.py	Sun Aug 30 13:58:11 2015 -0700
+++ b/mercurial/revlog.py	Sun Aug 30 14:03:32 2015 -0700
@@ -210,6 +210,7 @@
         self._chunkcache = (0, '')
         self._chunkcachesize = 65536
         self._maxchainlen = None
+        self._aggressivemergedeltas = False
         self.index = []
         self._pcache = {}
         self._nodecache = {nullid: nullrev}
@@ -227,6 +228,8 @@
                 self._chunkcachesize = opts['chunkcachesize']
             if 'maxchainlen' in opts:
                 self._maxchainlen = opts['maxchainlen']
+            if 'aggressivemergedeltas' in opts:
+                self._aggressivemergedeltas = opts['aggressivemergedeltas']
 
         if self._chunkcachesize <= 0:
             raise RevlogError(_('revlog chunk cache size %r is not greater '
@@ -1343,15 +1346,34 @@
         # should we try to build a delta?
         if prev != nullrev:
             if self._generaldelta:
-                # Pick whichever parent is closer to us (to minimize the
-                # chance of having to build a fulltext). Since
-                # nullrev == -1, any non-merge commit will always pick p1r.
-                drev = p2r if p2r > p1r else p1r
-                d = builddelta(drev)
-                # If the chosen delta will result in us making a full text,
-                # give it one last try against prev.
-                if drev != prev and not self._isgooddelta(d, textlen):
-                    d = builddelta(prev)
+                if p2r != nullrev and self._aggressivemergedeltas:
+                    d = builddelta(p1r)
+                    d2 = builddelta(p2r)
+                    p1good = self._isgooddelta(d, textlen)
+                    p2good = self._isgooddelta(d2, textlen)
+                    if p1good and p2good:
+                        # If both are good deltas, choose the smallest
+                        if d2[1] < d[1]:
+                            d = d2
+                    elif p2good:
+                        # If only p2 is good, use it
+                        d = d2
+                    elif p1good:
+                        pass
+                    else:
+                        # Neither is good, try against prev to hopefully save us
+                        # a fulltext.
+                        d = builddelta(prev)
+                else:
+                    # Pick whichever parent is closer to us (to minimize the
+                    # chance of having to build a fulltext). Since
+                    # nullrev == -1, any non-merge commit will always pick p1r.
+                    drev = p2r if p2r > p1r else p1r
+                    d = builddelta(drev)
+                    # If the chosen delta will result in us making a full text,
+                    # give it one last try against prev.
+                    if drev != prev and not self._isgooddelta(d, textlen):
+                        d = builddelta(prev)
             else:
                 d = builddelta(prev)
             dist, l, data, base, chainbase, chainlen, compresseddeltalen = d

--- a/tests/test-generaldelta.t	Sun Aug 30 13:58:11 2015 -0700
+++ b/tests/test-generaldelta.t	Sun Aug 30 14:03:32 2015 -0700
@@ -69,3 +69,37 @@
      rev    offset  length   base linkrev nodeid       p1           p2
        0         0       3      0       1 1406e7411862 000000000000 000000000000
 
+  $ cd ..
+
+Test format.aggressivemergedeltas
+
+  $ hg init --config format.generaldelta=1 aggressive
+  $ cd aggressive
+  $ touch a b c d e
+  $ hg commit -Aqm side1
+  $ hg up -q null
+  $ touch x y
+  $ hg commit -Aqm side2
+
+- Verify non-aggressive merge uses p1 (commit 1) as delta parent
+  $ hg merge -q 0
+  $ hg commit -q -m merge
+  $ hg debugindex -m
+     rev    offset  length  delta linkrev nodeid       p1           p2
+       0         0      59     -1       0 8dde941edb6e 000000000000 000000000000
+       1        59      59     -1       1 315c023f341d 000000000000 000000000000
+       2       118      65      1       2 2ab389a983eb 315c023f341d 8dde941edb6e
+
+  $ hg strip -q -r . --config extensions.strip=
+
+- Verify aggressive merge uses p2 (commit 0) as delta parent
+  $ hg up -q -C 1
+  $ hg merge -q 0
+  $ hg commit -q -m merge --config format.aggressivemergedeltas=True
+  $ hg debugindex -m
+     rev    offset  length  delta linkrev nodeid       p1           p2
+       0         0      59     -1       0 8dde941edb6e 000000000000 000000000000
+       1        59      59     -1       1 315c023f341d 000000000000 000000000000
+       2       118      62      0       2 2ab389a983eb 315c023f341d 8dde941edb6e
+
+  $ cd ..