help_unknown_ref(): check for refname ambiguity

When the user asks to merge "foo" and we suggest "origin/foo" instead,
we do so by simply chopping off "refs/remotes/" from the front of the
suggested ref. This is usually fine, but it's possible that the
resulting name is ambiguous (e.g., you have "refs/heads/origin/foo",
too).

Let's use shorten_unambiguous_ref() to do this the right way, which
should usually yield the same "origin/foo", but "remotes/origin/foo" if
necessary.

Note that in this situation there may be other options (e.g., we could
suggest "heads/origin/foo" as well). I'll leave that up for debate; the
focus here is just to avoid giving advice that does not actually do what
we expect.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

help.c

@@ -761,12 +761,12 @@ static int append_similar_ref(const char *refname, const struct object_id *oid,
 {
 	struct similar_ref_cb *cb = (struct similar_ref_cb *)(cb_data);
 	char *branch = strrchr(refname, '/') + 1;
-	const char *remote;
 
 	/* A remote branch of the same name is deemed similar */
-	if (skip_prefix(refname, "refs/remotes/", &remote) &&
+	if (starts_with(refname, "refs/remotes/") &&
 	    !strcmp(branch, cb->base_ref))
-		string_list_append(cb->similar_refs, remote);
+		string_list_append_nodup(cb->similar_refs,
+					 shorten_unambiguous_ref(refname, 1));
 	return 0;
 }
 

t/t7600-merge.sh

@@ -836,4 +836,10 @@ test_expect_success 'merge suggests matching remote refname' '
 	grep origin/not-local stderr
 '
 
+test_expect_success 'suggested names are not ambiguous' '
+	git update-ref refs/heads/origin/not-local HEAD &&
+	test_must_fail git merge not-local 2>stderr &&
+	grep remotes/origin/not-local stderr
+'
+
 test_done