remote-hg: implement custom push()

The one from mercurial does a ton of things we are not interested in,
and we need some special modifications which are impossible otherwise.

Most of the code is borrowed from Mercurial, and cleaned up, but should
be functionally the same for our purposes, except that multiple heads
are not detected.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/remote-helpers/git-remote-hg

@@ -12,7 +12,7 @@
 # For remote repositories a local clone is stored in
 # "$GIT_DIR/hg/origin/clone/.hg/".
 
-from mercurial import hg, ui, bookmarks, context, encoding, node, error, extensions
+from mercurial import hg, ui, bookmarks, context, encoding, node, error, extensions, discovery
 
 import re
 import sys
@@ -854,6 +854,46 @@ def write_tag(repo, tag, node, msg, author):
 
     return tagnode
 
+def push_unsafe(repo, remote, parsed_refs, p_revs):
+
+    force = force_push
+
+    fci = discovery.findcommonincoming
+    commoninc = fci(repo, remote, force=force)
+    common, _, remoteheads = commoninc
+
+    # TODO checkheads
+
+    cg = repo.getbundle('push', heads=list(p_revs), common=common)
+
+    unbundle = remote.capable('unbundle')
+    if unbundle:
+        if force:
+            remoteheads = ['force']
+        return remote.unbundle(cg, remoteheads, 'push')
+    else:
+        return remote.addchangegroup(cg, 'push', repo.url())
+
+def push(repo, remote, parsed_refs, p_revs):
+    if hasattr(remote, 'canpush') and not remote.canpush():
+        print "error cannot push"
+
+    if not p_revs:
+        # nothing to push
+        return
+
+    lock = None
+    unbundle = remote.capable('unbundle')
+    if not unbundle:
+        lock = remote.lock()
+    try:
+        ret = push_unsafe(repo, remote, parsed_refs, p_revs)
+    finally:
+        if lock is not None:
+            lock.release()
+
+    return ret
+
 def do_export(parser):
     global parsed_refs, bmarks, peer
 
@@ -919,7 +959,7 @@ def do_export(parser):
             continue
 
     if peer:
-        parser.repo.push(peer, force=force_push, newbranch=True, revs=list(p_revs))
+        push(parser.repo, peer, parsed_refs, p_revs)
 
         # update remote bookmarks
         remote_bmarks = peer.listkeys('bookmarks')