remote-bzr: add support to push merges

remote-bzr: add support to push merges

In order to do that, we need to store the marks of every file, so that
they can be fetched when needed. Unfortunately we can't tell bazaar that
nothing changed, we need to send the data so that it can figure it out
by itself.

And since it will be requesting a bunch of information by the file_id,
it's better to have a helper dict (rev_files), so that we can fetch it
quickly.

Signed-off-by: Felipe Contreras <redacted>
Signed-off-by: Junio C Hamano <redacted>