merge-base --independent to print reduced parent list in a merge

While show-branch --independent does not support more than MAX_REVS
revs, git internally supports more with a different algorithm.
Expose that functionality as "git merge-base --independent".

This should help scripts to catch up with builtin merge in supporting
dodecapus.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-merge-base.txt

@@ -8,7 +8,9 @@ git-merge-base - Find as good common ancestors as possible for a merge
 
 SYNOPSIS
 --------
+[verse]
 'git merge-base' [-a|--all] [--octopus] <commit> <commit>...
+'git merge-base' --independent <commit>...
 
 DESCRIPTION
 -----------
@@ -42,6 +44,13 @@ OPTIONS
 	in preparation for an n-way merge.  This mimics the behavior
 	of 'git show-branch --merge-base'.
 
+--independent::
+	Instead of printing merge bases, print a minimal subset of
+	the supplied commits with the same ancestors.  In other words,
+	among the commits given, list those which cannot be reached
+	from any other.  This mimics the behavior of 'git show-branch
+	--independent'.
+
 DISCUSSION
 ----------
 

builtin/merge-base.c

@@ -24,6 +24,7 @@ static int show_merge_base(struct commit **rev, int rev_nr, int show_all)
 
 static const char * const merge_base_usage[] = {
 	"git merge-base [-a|--all] [--octopus] <commit> <commit>...",
+	"git merge-base --independent <commit>...",
 	NULL
 };
 
@@ -41,15 +42,19 @@ static struct commit *get_commit_reference(const char *arg)
 	return r;
 }
 
-static int show_octopus_merge_bases(int count, const char **args, int show_all)
+static int handle_octopus(int count, const char **args, int reduce, int show_all)
 {
 	struct commit_list *revs = NULL;
 	struct commit_list *result;
 	int i;
 
-	for (i = count - 1; i >= 0; i++)
+	if (reduce)
+		show_all = 1;
+
+	for (i = count - 1; i >= 0; i--)
 		commit_list_insert(get_commit_reference(args[i]), &revs);
-	result = get_octopus_merge_bases(revs);
+
+	result = reduce ? reduce_heads(revs) : get_octopus_merge_bases(revs);
 
 	if (!result)
 		return 1;
@@ -70,20 +75,24 @@ int cmd_merge_base(int argc, const char **argv, const char *prefix)
 	int rev_nr = 0;
 	int show_all = 0;
 	int octopus = 0;
+	int reduce = 0;
 
 	struct option options[] = {
 		OPT_BOOLEAN('a', "all", &show_all, "output all common ancestors"),
 		OPT_BOOLEAN(0, "octopus", &octopus, "find ancestors for a single n-way merge"),
+		OPT_BOOLEAN(0, "independent", &reduce, "list revs not reachable from others"),
 		OPT_END()
 	};
 
 	git_config(git_default_config, NULL);
 	argc = parse_options(argc, argv, prefix, options, merge_base_usage, 0);
-	if (!octopus && argc < 2)
+	if (!octopus && !reduce && argc < 2)
 		usage_with_options(merge_base_usage, options);
+	if (reduce && (show_all || octopus))
+		die("--independent cannot be used with other options");
 
-	if (octopus)
-		return show_octopus_merge_bases(argc, argv, show_all);
+	if (octopus || reduce)
+		return handle_octopus(argc, argv, reduce, show_all);
 
 	rev = xmalloc(argc * sizeof(*rev));
 	while (argc-- > 0)

t/t6010-merge-base.sh

@@ -3,7 +3,7 @@
 # Copyright (c) 2005 Junio C Hamano
 #
 
-test_description='Merge base computation.
+test_description='Merge base and parent list computation.
 '
 
 . ./test-lib.sh
@@ -75,6 +75,26 @@ test_expect_success 'merge-base G H' '
 	test_cmp expected actual.sb
 '
 
+test_expect_success 'merge-base/show-branch --independent' '
+	git name-rev "$H" >expected1 &&
+	git name-rev "$H" "$G" >expected2 &&
+
+	parents=$(git merge-base --independent H) &&
+	git name-rev $parents >actual1.mb &&
+	parents=$(git merge-base --independent A H G) &&
+	git name-rev $parents >actual2.mb &&
+
+	parents=$(git show-branch --independent H) &&
+	git name-rev $parents >actual1.sb &&
+	parents=$(git show-branch --independent A H G) &&
+	git name-rev $parents >actual2.sb &&
+
+	test_cmp expected1 actual1.mb &&
+	test_cmp expected2 actual2.mb &&
+	test_cmp expected1 actual1.sb &&
+	test_cmp expected2 actual2.sb
+'
+
 test_expect_success 'unsynchronized clocks' '
 	# This test is to demonstrate that relying on timestamps in a distributed
 	# SCM to provide a _consistent_ partial ordering of commits leads to
@@ -125,6 +145,23 @@ test_expect_success 'unsynchronized clocks' '
 	test_cmp expected actual.all
 '
 
+test_expect_success '--independent with unsynchronized clocks' '
+	IB=$(doit 0 IB) &&
+	I1=$(doit -10 I1 $IB) &&
+	I2=$(doit  -9 I2 $I1) &&
+	I3=$(doit  -8 I3 $I2) &&
+	I4=$(doit  -7 I4 $I3) &&
+	I5=$(doit  -6 I5 $I4) &&
+	I6=$(doit  -5 I6 $I5) &&
+	I7=$(doit  -4 I7 $I6) &&
+	I8=$(doit  -3 I8 $I7) &&
+	IH=$(doit  -2 IH $I8) &&
+
+	echo $IH >expected &&
+	git merge-base --independent IB IH >actual &&
+	test_cmp expected actual
+'
+
 test_expect_success 'merge-base for octopus-step (setup)' '
 	# Another set to demonstrate base between one commit and a merge
 	# in the documentation.