diff: use skip_to_optional_arg()

Let's simplify diff option parsing using skip_to_optional_arg().
Signed-off-by: NChristian Couder <chriscool@tuxfamily.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

diff.c

@@ -4508,17 +4508,12 @@ int diff_opt_parse(struct diff_options *options,
 		options->output_format |= DIFF_FORMAT_NUMSTAT;
 	else if (!strcmp(arg, "--shortstat"))
 		options->output_format |= DIFF_FORMAT_SHORTSTAT;
-	else if (!strcmp(arg, "-X") || !strcmp(arg, "--dirstat"))
-		return parse_dirstat_opt(options, "");
-	else if (skip_prefix(arg, "-X", &arg))
-		return parse_dirstat_opt(options, arg);
-	else if (skip_prefix(arg, "--dirstat=", &arg))
+	else if (skip_prefix(arg, "-X", &arg) ||
+		 skip_to_optional_arg(arg, "--dirstat", &arg))
 		return parse_dirstat_opt(options, arg);
 	else if (!strcmp(arg, "--cumulative"))
 		return parse_dirstat_opt(options, "cumulative");
-	else if (!strcmp(arg, "--dirstat-by-file"))
-		return parse_dirstat_opt(options, "files");
-	else if (skip_prefix(arg, "--dirstat-by-file=", &arg)) {
+	else if (skip_to_optional_arg(arg, "--dirstat-by-file", &arg)) {
 		parse_dirstat_opt(options, "files");
 		return parse_dirstat_opt(options, arg);
 	}
@@ -4540,13 +4535,13 @@ int diff_opt_parse(struct diff_options *options,
 		return stat_opt(options, av);
 
 	/* renames options */
-	else if (starts_with(arg, "-B") || starts_with(arg, "--break-rewrites=") ||
-		 !strcmp(arg, "--break-rewrites")) {
+	else if (starts_with(arg, "-B") ||
+		 skip_to_optional_arg(arg, "--break-rewrites", NULL)) {
 		if ((options->break_opt = diff_scoreopt_parse(arg)) == -1)
 			return error("invalid argument to -B: %s", arg+2);
 	}
-	else if (starts_with(arg, "-M") || starts_with(arg, "--find-renames=") ||
-		 !strcmp(arg, "--find-renames")) {
+	else if (starts_with(arg, "-M") ||
+		 skip_to_optional_arg(arg, "--find-renames", NULL)) {
 		if ((options->rename_score = diff_scoreopt_parse(arg)) == -1)
 			return error("invalid argument to -M: %s", arg+2);
 		options->detect_rename = DIFF_DETECT_RENAME;
@@ -4554,8 +4549,8 @@ int diff_opt_parse(struct diff_options *options,
 	else if (!strcmp(arg, "-D") || !strcmp(arg, "--irreversible-delete")) {
 		options->irreversible_delete = 1;
 	}
-	else if (starts_with(arg, "-C") || starts_with(arg, "--find-copies=") ||
-		 !strcmp(arg, "--find-copies")) {
+	else if (starts_with(arg, "-C") ||
+		 skip_to_optional_arg(arg, "--find-copies", NULL)) {
 		if (options->detect_rename == DIFF_DETECT_COPY)
 			options->flags.find_copies_harder = 1;
 		if ((options->rename_score = diff_scoreopt_parse(arg)) == -1)