commit_packed_refs(): use reference iteration

commit_packed_refs(): use reference iteration

Use reference iteration rather than do_for_each_entry_in_dir() in the
definition of commit_packed_refs().

Note that an internal consistency check that was previously done in
`write_packed_entry_fn()` is not there anymore. This is actually an
improvement:

The old error message was emitted when there is an entry in the
packed-ref cache that is not `REF_KNOWS_PEELED`, and when we attempted
to peel the reference, the result was `PEEL_INVALID`,
`PEEL_IS_SYMREF`, or `PEEL_BROKEN`. Since a packed ref cannot be a
symref, `PEEL_IS_SYMREF` and `PEEL_BROKEN` can be ruled out. So we're
left with `PEEL_INVALID`.

An entry without `REF_KNOWS_PEELED` can get into the packed-refs cache
in the following two ways:

* The reference was read from a `packed-refs` file that didn't have
  the `fully-peeled` attribute. In that case, we *don't want* to emit
  an error, because the broken value is presumably a stale value of
  the reference that is now masked by a loose version of the same
  reference (which we just don't happen to be packing this time). This
  is a perfectly legitimate situation and doesn't indicate that the
  repository is corrupt. The old code incorrectly emits an error
  message in this case. (It was probably never reported as a bug
  because this scenario is rare.)

* The reference was a loose reference that was just added to the
  packed ref cache by `files_packed_refs()` via
  `pack_if_possible_fn()` in preparation for being packed. The latter
  function refuses to pack a reference for which
  `entry_resolves_to_object()` returns false, and otherwise calls
  `peel_entry()` itself and checks the return value. So an entry added
  this way should always have `REF_KNOWS_PEELED` and shouldn't trigger
  the error message in either the old code or the new.

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