mailinfo: simplify parsing of header values
Our code to parse header values first checks to see if a line starts
with a header, and then manually skips past the matched string to find
the value. We can do this all in one step by modeling after
skip_prefix(), which returns a pointer into the string after the
parsing.
This lets us remove some repeated strings, and will also enable us to
parse more flexibly in a future patch.
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>