diff-merges: introduce --diff-merges=on

Introduce the notion of default diff format for merges, and the option
"on" to select it. The default format is "separate" and can't yet
be changed, so effectively "on" is just a synonym for "separate"
for now. Add corresponding test to t4013.

This is in preparation for introducing log.diffMerges configuration
option that will let --diff-merges=on to be configured to any
supported format.

Signed-off-by: Sergey Organov <sorganov@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff-merges.c

@@ -2,6 +2,11 @@
 
 #include "revision.h"
 
+typedef void (*diff_merges_setup_func_t)(struct rev_info *);
+static void set_separate(struct rev_info *revs);
+
+static diff_merges_setup_func_t set_to_default = set_separate;
+
 static void suppress(struct rev_info *revs)
 {
 	revs->separate_merges = 0;
@@ -66,6 +71,8 @@ static void set_diff_merges(struct rev_info *revs, const char *optarg)
 		set_combined(revs);
 	else if (!strcmp(optarg, "cc") || !strcmp(optarg, "dense-combined"))
 		set_dense_combined(revs);
+	else if (!strcmp(optarg, "on"))
+		set_to_default(revs);
 	else
 		die(_("unknown value for --diff-merges: %s"), optarg);
 

t/t4013-diff-various.sh

@@ -452,6 +452,14 @@ diff-tree --stat --compact-summary initial mode
 diff-tree -R --stat --compact-summary initial mode
 EOF
 
+test_expect_success 'log --diff-merges=on matches --diff-merges=separate' '
+	git log -p --diff-merges=separate master >result &&
+	process_diffs result >expected &&
+	git log -p --diff-merges=on master >result &&
+	process_diffs result >actual &&
+	test_cmp expected actual
+'
+
 test_expect_success 'log -S requires an argument' '
 	test_must_fail git log -S
 '