fetch: set remote/HEAD if it does not exist

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>