scripts/get_maintainer.pl: handle file names beginning with ./

The problem is that get_maintainer.pl doesn't work if you have a ./
prefix on the filename.  For example, if you type:

  ./scripts/get_maintainer.pl -f ./drivers/usb/usb-skeleton.c

then the current code only includes LKML and people from the git log, it
doesn't include Greg or the linux-usb list.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>