packed_ref_cache: keep the `packed-refs` file mmapped if possible

Keep a copy of the `packed-refs` file contents in memory for as long
as a `packed_ref_cache` object is in use:

* If the system allows it, keep the `packed-refs` file mmapped.

* If not (either because the system doesn't support `mmap()` at all,
  or because a file that is currently mmapped cannot be replaced via
  `rename()`), then make a copy of the file's contents in
  heap-allocated space, and keep that around instead.

We base the choice of behavior on a new build-time switch,
`MMAP_PREVENTS_DELETE`. By default, this switch is set for Windows
variants.

After this commit, `MMAP_NONE` and `MMAP_TEMPORARY` are still handled
identically. But the next commit will introduce a difference.

This whole change is still pointless, because we only read the
`packed-refs` file contents immediately after instantiating the
`packed_ref_cache`. But that will soon change.

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

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