xdiff: use git-compat-util

xdiff: use git-compat-util

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