Re: [PATCH 20/20] packed-backend.c: rename a bunch of things and update comments

> We've made huge changes to this file, and some of the old names and
> comments are no longer very fitting. So rename a bunch of things:
>
> * `struct packed_ref_cache` → `struct snapshot`
> * `acquire_packed_ref_cache()` → `acquire_snapshot()`
> * `release_packed_ref_buffer()` → `clear_snapshot_buffer()`
> * `release_packed_ref_cache()` → `release_snapshot()`
> * `clear_packed_ref_cache()` → `clear_snapshot()`
> * `struct packed_ref_entry` → `struct snapshot_record`
> * `cmp_packed_ref_entries()` → `cmp_packed_ref_records()`
> * `cmp_entry_to_refname()` → `cmp_record_to_refname()`
> * `sort_packed_refs()` → `sort_snapshot()`
> * `read_packed_refs()` → `create_snapshot()`
> * `validate_packed_ref_cache()` → `validate_snapshot()`
> * `get_packed_ref_cache()` → `get_snapshot()`
> * Renamed local variables and struct members accordingly.
>
> Also update a bunch of comments to reflect the renaming and the
> accumulated changes that the code has undergone.
>
> Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>