mailinfo: simplify parsing of header values

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 <redacted>
Signed-off-by: Junio C Hamano <redacted>