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 --git a/Documentation/diff-options.txt b/Documentation/diff-options.txt
index 2a0275e..ec6811c 100644
--- a/Documentation/diff-options.txt
+++ b/Documentation/diff-options.txt
@@ -69,6 +69,10 @@
 	changeset, not just the files that contain the change
 	in <string>.
 
+--pickaxe-regex::
+	Make the <string> not a plain string but an extended POSIX
+	regex to match.
+
 -O<orderfile>::
 	Output the patch in the order specified in the
 	<orderfile>, which has one shell glob pattern per line.