diff: fix --merge-base with annotated tags

Checking early for OBJ_COMMIT excludes other objects that can be
resolved to commits, like annotated tags.  If we remove it, annotated
tags will be resolved and handled just fine by
lookup_commit_reference(), and if we are given something that can't be
resolved to a commit, we'll still get a useful error message, e.g.:

> error: object 21ab162211ac3ef13c37603ca88b27e9c7e0d40b is a tree, not a commit
> fatal: no merge base found

Signed-off-by: Alyssa Ross <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: alyssais

diff-lib.c

@@ -572,8 +572,6 @@ 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" '