shortlog: match both "Author:" and "author" on stdin

shortlog: match both "Author:" and "author" on stdin

The original git-shortlog could read both the normal "git
log" output as well as "git log --format=raw". However, when
it was converted to C by b8ec592 (Build in shortlog,
2006-10-22), the trailing colon became mandatory, and we no
longer matched the raw output.

Given the amount of intervening time without any bug
reports, it's probable that nobody cares. But it's
relatively easy to fix, and the end result is hopefully more
readable than the original.

Note that this no longer matches "author: ", which we did
before, but that has never been a format generated by git.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>