Re: [PATCH v2 13/21] 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.
> 
> 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>
> ---
>  Makefile              |  10 +++
>  config.mak.uname      |   3 +
>  refs/packed-backend.c | 184 ++++++++++++++++++++++++++++++++++++++------------
>  3 files changed, 155 insertions(+), 42 deletions(-)
> 
> diff --git a/Makefile b/Makefile
> index f2bb7f2f63..7a49f99c4f 100644
> [...]
> diff --git a/refs/packed-backend.c b/refs/packed-backend.c
> index 0fe41a7203..4981516f1e 100644
> --- a/refs/packed-backend.c
> +++ b/refs/packed-backend.c
> [...]
> @@ -304,6 +371,61 @@ struct ref_iterator *mmapped_ref_iterator_begin(
>  	return ref_iterator;
>  }
>  
> +/*
> + * Depending on `mmap_strategy`, either mmap or read the contents of
> + * the `packed-refs` file into the `packed_refs` instance. Return 1 if
> + * the file existed and was read, or 0 if the file was absent. Die on
> + * errors.
> + */
> +static int load_contents(struct packed_ref_cache *packed_refs)
> +{
> +	int fd;
> +	struct stat st;
> +	size_t size, bytes_read;