config: support values longer than 1023 bytes

config: support values longer than 1023 bytes

parse_value in config.c has a static buffer of 1024 bytes that it
parse the value into. This can sometimes be a problem when a
config file contains very long values.

It's particularly amusing that git-config already is able to write
such files, so it should probably be able to read them as well.

Fix this by using a strbuf instead of a fixed-size buffer.

Signed-off-by: Erik Faye-Lund <kusmabite@gmail.com>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>