In a repository cloned from somewhere else, you typically have a
symbolic ref refs/remotes/origin/HEAD pointing at the 'master'
remote-tracking ref that is next to it.  When fetching into such a
repository with "git fetch --mirror" from another repository that
was similarly cloned, the implied wildcard refspec refs/*:refs/*
will end up asking to update refs/remotes/origin/HEAD with the
object at refs/remotes/origin/HEAD at the remote side, while asking
to update refs/remotes/origin/master the same way.  Depending on the
order the two updates happen, the latter one would find that the
value of the ref before it is updated has changed from what the code
expects.

When the user asks to update the underlying ref via the symbolic ref
explicitly without using a wildcard refspec, e.g. "git fetch $there
refs/heads/master:refs/remotes/origin/HEAD", we should still let him
do so, but when expanding wildcard refs, it will result in a more
intuitive outcome if we simply ignore local symbolic refs.

As the purpose of the symbolic ref refs/remotes/origin/HEAD is to
follow the ref it points at (e.g. refs/remotes/origin/master), its
value would change when the underlying ref is updated.

Earlier commit da3efdb1

 (receive-pack: detect aliased updates which
can occur with symrefs, 2010-04-19) fixed a similar issue for "git
push".

Signed-off-by: Junio C Hamano <gitster@pobox.com>