git diff: support "-U" and "--unified" options properly

We used to parse "-U" and "--unified" as part of the GIT_DIFF_OPTS
environment variable, but strangely enough we would _not_ parse them as
part of the normal diff command line (where we only accepted "-u").

This adds parsing of -U and --unified, both with an optional numeric
argument. So now you can just say

	git diff --unified=5

to get a unified diff with a five-line context, instead of having to do
something silly like

	GIT_DIFF_OPTS="--unified=5" git diff -u

(that silly format does continue to still work, of course).

Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

diff.h

@@ -32,6 +32,7 @@ struct diff_options {
 		 full_index:1,
 		 silent_on_remove:1,
 		 find_copies_harder:1;
+	int context;
 	int break_opt;
 	int detect_rename;
 	int line_termination;