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 <[email protected]>
Cc: Marti Raudsepp <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>