repack_without_ref(): write peeled refs in the rewritten file

repack_without_ref(): write peeled refs in the rewritten file

When a reference that existed in the packed-refs file is deleted, the
packed-refs file must be rewritten.  Previously, the file was
rewritten without any peeled refs, even if the file contained peeled
refs when it was read.  This was not a bug, because the packed-refs
file header didn't claim that the file contained peeled values.  But
it had a performance cost, because the repository would lose the
benefit of having precomputed peeled references until pack-refs was
run again.

Teach repack_without_ref() to write peeled refs to the packed-refs
file (regardless of whether they were present in the old version of
the file).

This means that if the old version of the packed-refs file was not
fully peeled, then repack_without_ref() will have to peel references.
To avoid the expense of reading lots of loose references, we take two
shortcuts relative to pack-refs:

* If the peeled value of a reference is already known (i.e., because
  it was read from the old version of the packed-refs file), then
  output that peeled value again without any checks.  This is the
  usual code path and should avoid any noticeable overhead.  (This is
  different than pack-refs, which always re-peels references.)

* We don't verify that the packed ref is still current.  It could be
  that a packed references is overridden by a loose reference, in
  which case the packed ref is no longer needed and might even refer
  to an object that has been garbage collected.  But we don't check;
  instead, we just try to peel all references.  If peeling is
  successful, the peeled value is written out (even though it might
  not be needed any more); if not, then the reference is silently
  omitted from the output.

The extra overhead of peeling references in repack_without_ref()
should only be incurred the first time the packed-refs file is written
by a version of Git that knows about the "fully-peeled" attribute.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>