determine_author_info(): reuse parsing functions

determine_author_info(): reuse parsing functions

Rather than parsing the header manually to find the "author"
field, and then parsing its sub-parts, let's use
find_commit_header and split_ident_line. This is shorter and
easier to read, and should do a more careful parsing job.

For example, the current parser could find the end-of-email
right-bracket across a newline (for a malformed commit), and
calculate a bogus gigantic length for the date (by using
"eol - rb").

As a bonus, this also plugs a memory leak when we pull the
date field from an existing commit (we still leak the name
and email buffers, which will be fixed in a later commit).

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