Since the xdiff library was not originally part of Git, it does its own
system includes. Let's instead use git-compat-util, which has two
benefits:
1. It adjusts for any system-specific quirks in how or what we should
include (though xdiff's needs are light enough that this hasn't
been a problem in the past).
2. It lets us use wrapper functions like xmalloc().
Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>