git-sh-setup: refactor ident-parsing functions

git-sh-setup: refactor ident-parsing functions

The only ident-parsing function we currently provide is
get_author_ident_from_commit. This is not very
flexible for two reasons:

  1. It takes a commit as an argument, and can't read from
     commit headers saved on disk.

  2. It will only parse authors, not committers.

This patch provides a more flexible interface which will
parse multiple idents from a commit provide on stdin. We can
easily use it as a building block for the current function
to retain compatibility.

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