Teach diff about -b and -w flags

This adds -b (--ignore-space-change) and -w (--ignore-all-space) flags to
diff. The main part of the patch is teaching libxdiff about it.

[jc: renamed xdl_line_match() to xdl_recmatch() since the former is used
 for different purposes in xpatchi.c which is in the parts of the upstream
 source we do not use.]

Signed-off-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>

diff.h

@@ -46,6 +46,7 @@ struct diff_options {
 	int setup;
 	int abbrev;
 	const char *stat_sep;
+	long xdl_opts;
 
 	int nr_paths;
 	const char **paths;