Merge branch 'ar/diff-index-merge-base-fix'

"git diff --merge-base X other args..." insisted that X must be a
commit and errored out when given an annotated tag that peels to a
commit, but we only need it to be a committish.  This has been
corrected.

* ar/diff-index-merge-base-fix:
  diff: fix --merge-base with annotated tags

Author: gitster

diff-lib.c

@@ -571,8 +571,6 @@ void diff_get_merge_base(const struct rev_info *revs, struct object_id *mb)
 		struct object *obj = revs->pending.objects[i].item;
 		if (obj->flags)
 			die(_("--merge-base does not work with ranges"));
-		if (obj->type != OBJ_COMMIT)
-			die(_("--merge-base only works with commits"));
 	}
 
 	/*

t/t4068-diff-symmetric-merge-base.sh

@@ -34,7 +34,7 @@ test_expect_success setup '
 	echo c >c &&
 	git add c &&
 	git commit -m C &&
-	git tag commit-C &&
+	git tag -m commit-C commit-C &&
 	git merge -m D main &&
 	git tag commit-D &&
 	git checkout main &&
@@ -109,6 +109,13 @@ do
 		test_cmp expect actual
 	'
 
+	test_expect_success "$cmd --merge-base with annotated tag" '
+		git checkout main &&
+		git $cmd commit-C >expect &&
+		git $cmd --merge-base commit-C >actual &&
+		test_cmp expect actual
+	'
+
 	test_expect_success "$cmd --merge-base with one commit and unstaged changes" '
 		git checkout main &&
 		test_when_finished git reset --hard &&
@@ -143,7 +150,7 @@ do
 	test_expect_success "$cmd --merge-base with non-commit" '
 		git checkout main &&
 		test_must_fail git $cmd --merge-base main^{tree} 2>err &&
-		test_i18ngrep "fatal: --merge-base only works with commits" err
+		test_i18ngrep "is a tree, not a commit" err
 	'
 
 	test_expect_success "$cmd --merge-base with no merge bases and one commit" '
@@ -169,7 +176,7 @@ do
 
 	test_expect_success "$cmd --merge-base commit and non-commit" '
 		test_must_fail git $cmd --merge-base br2 main^{tree} 2>err &&
-		test_i18ngrep "fatal: --merge-base only works with commits" err
+		test_i18ngrep "is a tree, not a commit" err
 	'
 
 	test_expect_success "$cmd --merge-base with no merge bases and two commits" '