[MERGE] Fix handling of old merge directives with stricter get_parent_map

[John Arbash Meinel]

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

Aaron Bentley wrote:
| Hi all,
|
| It turns out that revno 3434, which made it illegal to pass None to
| get_parent_map, exposed a latent bug in Merger.from_mergeable.  Format 1
| merge directives did not provide a base_revision_id, but from_mergeable
| assumes one is present (which is always true for Format 2 merge directives).
|
| No bug number for this.  I just ran it down now.
|
| Aaron

BB:approve

John
=:->
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.4.9 (Cygwin)
Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org

iEYEARECAAYFAkg3POEACgkQJdeBCYSNAAPbBQCgjEgRGYObYFqexfmk3KJta4Eb
FRsAn2/Mp5wbH0FA1zsPTZCPFhMPgUGW
=SiF8
-----END PGP SIGNATURE-----