scripts/get_maintainer.pl: warn on missing git or git repository

support older versions of grep (use -E not -P)
no need to return data in routine recent_git_signoffs

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>