git-svnimport: Improved detection of merges.

The regexes detecting merges (while still relying on the commit messages,
though) have been improved to catch saner (and hopefully more) messages. The
old regex was so generic that it often matched something else and missed the
actual merge-message.
Also, the regex given with the `-M' commandline-option is checked first:
Explicitely given regexes should be considered better than the builtin ones,
and should therefore be given a chance to match a message first.

Signed-off-by: Junio C Hamano <junkio@cox.net>