patch-id.c: use strbuf instead of a fixed buffer

patch-id.c: use strbuf instead of a fixed buffer

get_one_patchid() uses a rather dumb heuristic to determine if the
passed buffer is part of the next commit. Whenever the first 40 bytes
are a valid hexadecimal sha1 representation, get_one_patchid() returns
next_sha1.

Once the current line is longer than the fixed buffer, this will break
(provided the additional bytes make a valid hexadecimal sha1). As a result
patch-id returns incorrect results. Instead, use strbuf and read one line
at a time.

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Michael Schubert <mschub@elegosoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>