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);

diff.c

@@ -558,7 +558,7 @@ static void builtin_diff(const char *name_a,
 
 		ecbdata.label_path = lbl;
 		xpp.flags = XDF_NEED_MINIMAL;
-		xecfg.ctxlen = 3;
+		xecfg.ctxlen = o->context;
 		xecfg.flags = XDL_EMIT_FUNCNAMES;
 		if (!diffopts)
 			;
@@ -1182,6 +1182,7 @@ void diff_setup(struct diff_options *options)
 	options->line_termination = '\n';
 	options->break_opt = -1;
 	options->rename_limit = -1;
+	options->context = 3;
 
 	options->change = diff_change;
 	options->add_remove = diff_addremove;
@@ -1222,11 +1223,60 @@ int diff_setup_done(struct diff_options *options)
 	return 0;
 }
 
+int opt_arg(const char *arg, int arg_short, const char *arg_long, int *val)
+{
+	char c, *eq;
+	int len;
+
+	if (*arg != '-')
+		return 0;
+	c = *++arg;
+	if (!c)
+		return 0;
+	if (c == arg_short) {
+		c = *++arg;
+		if (!c)
+			return 1;
+		if (val && isdigit(c)) {
+			char *end;
+			int n = strtoul(arg, &end, 10);
+			if (*end)
+				return 0;
+			*val = n;
+			return 1;
+		}
+		return 0;
+	}
+	if (c != '-')
+		return 0;
+	arg++;
+	eq = strchr(arg, '=');
+	if (eq)
+		len = eq - arg;
+	else
+		len = strlen(arg);
+	if (!len || strncmp(arg, arg_long, len))
+		return 0;
+	if (eq) {
+		int n;
+		char *end;
+		if (!isdigit(*++eq))
+			return 0;
+		n = strtoul(eq, &end, 10);
+		if (*end)
+			return 0;
+		*val = n;
+	}
+	return 1;
+}
+
 int diff_opt_parse(struct diff_options *options, const char **av, int ac)
 {
 	const char *arg = av[0];
 	if (!strcmp(arg, "-p") || !strcmp(arg, "-u"))
 		options->output_format = DIFF_FORMAT_PATCH;
+	else if (opt_arg(arg, 'U', "unified", &options->context))
+		options->output_format = DIFF_FORMAT_PATCH;
 	else if (!strcmp(arg, "--patch-with-raw")) {
 		options->output_format = DIFF_FORMAT_PATCH;
 		options->with_raw = 1;

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;