scripts/get_maintainer.pl: add .mailmap use, shell and email cleanups

Add reading and using .mailmap file if it exists
Convert address entries in .mailmap to first encountered address
Don't terminate shell commands with \n
Strip characters found after sign-offs by: name <address> [stripped]

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>