"git log frotz" can DWIM to "refs/remotes/frotz/HEAD", but in the remote
access context, "git fetch frotz" to fetch what the other side happened to
have fetched from what it calls 'frotz' (which may not have any relation
to what we consider is 'frotz') the last time would not make much sense,
so the fetch rules table did not include "refs/remotes/%.*s/HEAD".

When the user really wants to, "git fetch $there remotes/frotz/HEAD" would
let her do so anyway, so this is not about safety or security; it merely
is about confusion avoidance and discouraging meaningless usage.

Specifically, it is _not_ about ambiguity avoidance. A name that would
become ambiguous if we use the same rules table for both fetch and local
rev-parse would be ambiguous locally at the remote side.

So for the same reason as we added rule to allow "git fetch $there v1.0"
instead of "git fetch $there tags/v1.0" in the previous commit, here is a
bit longer rope for the users, which incidentally simplifies our code.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>