Support for pickaxe matching regular expressions

git-diff-* --pickaxe-regex will change the -S pickaxe to match
POSIX extended regular expressions instead of fixed strings.

The regex.h library is a rather stupid interface and I like pcre too, but
with any luck it will be everywhere we will want to run Git on, it being
POSIX.2 and all. I'm not sure if we can expect platforms like AIX to
conform to POSIX.2 or if win32 has regex.h. We might add a flag to
Makefile if there is a portability trouble potential.

Signed-off-by: Petr Baudis <pasky@suse.cz>

diff.h

@@ -102,6 +102,7 @@ extern int diff_setup_done(struct diff_options *);
 #define DIFF_DETECT_COPY	2
 
 #define DIFF_PICKAXE_ALL	1
+#define DIFF_PICKAXE_REGEX	2
 
 extern void diffcore_std(struct diff_options *);