Optimize peel_ref for the current ref of a for_each_ref callback

Optimize peel_ref for the current ref of a for_each_ref callback

Currently the only caller of peel_ref is show-ref, which is using
this function to show the peeled tag information if it is available
from an existing packed-refs file.  The call happens during the
for_each_ref callback function, so we have the proper struct ref_list
already on the call stack but it is not easily available to return
the peeled information to the caller.

We now save the current struct ref_list item before calling back
into the callback function so that future calls to peel_ref from
within the callback function can quickly access the current ref.
Doing so will save us an lstat() per ref processed as we no longer
have to check the filesystem to see if the ref exists as a loose
file or is packed.  This current ref caching also saves a linear
scan of the cached packed refs list.

As a micro-optimization we test the address of the passed ref name
against the current_ref->name before we go into the much more costly
strcmp().  Nearly any caller of peel_ref will be passing us the same
string do_for_each_ref passed them, which is current_ref->name.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>