fetch: set remote/HEAD if it does not exist

When cloning a repository remote/HEAD is created, but when the user
creates a repository with git init, and later adds a remote, remote/HEAD
is only created if the user explicitly runs a variant of "remote
set-head". Attempt to set remote/HEAD during fetch, if the user does not
have it already set. Silently ignore any errors.

Signed-off-by: Bence Ferdinandy <bence@ferdinandy.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5516-fetch-push.sh

@@ -1395,7 +1395,8 @@ test_expect_success 'fetch follows tags by default' '
 		git tag -m "annotated" tag &&
 		git for-each-ref >tmp1 &&
 		sed -n "p; s|refs/heads/main$|refs/remotes/origin/main|p" tmp1 |
-		sort -k 3 >../expect
+		sed -n "p; s|refs/heads/main$|refs/remotes/origin/HEAD|p"  |
+		sort -k 4 >../expect
 	) &&
 	test_when_finished "rm -rf dst" &&
 	git init dst &&