git-svn: bugfix and optimize the 'log' command

Revisions with long commit messages were being skipped, since
the 'git-svn-id' metadata line was at the end and git-log uses a
32k buffer to print the commits.

Also the last 'git-svn-id' metadata line in a commit is always
the valid one, so make sure we use that, as well.

Made the verbose flag work by passing the correct option switch
('--summary') to git-log.

Finally, optimize -r/--revision argument handling by passing
the appropriate limits to revision

Signed-off-by: Eric Wong <normalperson@yhbt.net>