git-svn: several graft-branches improvements

git-svn: several graft-branches improvements

The 'graft-branches' command can now analyze tree matches for
merge detection after commits are done, when --branch or
--branch-all-refs options are used.

We ensure that tree joins (--branch and --branch-all-refs
options) during commit time only add SVN parents that occurred
before the commit we're importing

Also fixed branch detection via merge messages, this manner of
merge detection (a la git-svnimport) is really all fuzzy, but at
least it actually works now :)

Add some new tests to go along with these fixes, too.

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>