diff-merges: implement new values for --diff-merges

We first implement new options as exact synonyms for their original
counterparts, to get all the infrastructure right, and keep functional
improvements for later commits.

The following values are implemented:

--diff-merges=	        old equivalent
first|first-parent    = --first-parent (only format implications)
sep|separate          = -m
comb|combined         = -c
dense| dense-combined = --cc

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

diff-merges.c

@@ -17,6 +17,12 @@ static void set_separate(struct rev_info *revs)
 	revs->separate_merges = 1;
 }
 
+static void set_first_parent(struct rev_info *revs)
+{
+	set_separate(revs);
+	revs->first_parent_merges = 1;
+}
+
 static void set_m(struct rev_info *revs)
 {
 	/*
@@ -44,11 +50,18 @@ static void set_dense_combined(struct rev_info *revs)
 
 static void set_diff_merges(struct rev_info *revs, const char *optarg)
 {
-	if (!strcmp(optarg, "off")) {
+	if (!strcmp(optarg, "off") || !strcmp(optarg, "none"))
 		suppress(revs);
-	} else {
+	else if (!strcmp(optarg, "first-parent"))
+		set_first_parent(revs);
+	else if (!strcmp(optarg, "separate"))
+		set_separate(revs);
+	else if (!strcmp(optarg, "combined"))
+		set_combined(revs);
+	else if (!strcmp(optarg, "dense-combined"))
+		set_dense_combined(revs);
+	else
 		die(_("unknown value for --diff-merges: %s"), optarg);
-	}
 }
 
 /*