fetch: limit shared symref check only for local branches

fetch: limit shared symref check only for local branches

This check was introduced in 8ee5d73137f (Fix fetch/pull when run without
--update-head-ok, 2008-10-13) in order to protect against replacing the ref
of the active branch by mistake, for example by running git fetch origin
master:master.

It was later extended in 8bc1f39f411 (fetch: protect branches checked out
in all worktrees, 2021-12-01) to scan all worktrees.

This operation is very expensive (takes about 30s in my repository) when
there are many tags or branches, and it is executed on every fetch, even if
no local heads are updated at all.

Limit it to protect only refs/heads/* to improve fetch performance.

Signed-off-by: Orgad Shaneh <orgads@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>