scripts/get_maintainer.pl: support multiple VCSs - add mercurial

Restructure a bit for multiple version control systems support.

Use a hash for each supported VCS that contains the commands
and patterns used to find commits, logs, and signers.

--git command line options are still used for hg except for
--git-since.  Use --hg-since instead.

The number of commits can differ for git and hg, so --rolestats
might be different.

Style changes: Use common push style push(@foo...), simplify a return

Bumped version to 0.23.
Signed-off-by: Joe Perches <joe@perches.com>
Cc: Marti Raudsepp <marti@juffo.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>