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>

combine-diff.c

@@ -608,6 +608,7 @@ static int show_patch_diff(struct combine_diff_path *elem, int num_parent,
 	int abbrev = opt->full_index ? 40 : DEFAULT_ABBREV;
 	mmfile_t result_file;
 
+	context = opt->context;
 	/* Read the result of merge first */
 	if (!working_tree_file)
 		result = grab_blob(elem->sha1, &result_size);