This patch implements a --num-commits (-n) flag for shorthand for
the range of hash~N..hash commits.

contrib/ChangeLog:

	* gcc-changelog/git_check_commit.py: Implement --num-commits.

Signed-off-by: Ken Matsui <kmatsui@gcc.gnu.org>
Reviewed-by: Patrick Palka <ppalka@redhat.com>
Reviewed-by: Jason Merrill <jason@redhat.com>