Re: [PATCH 6/6] fetch: make `--atomic` flag cover pruning of refs

>
> When fetching with the `--prune` flag we will delete any local
> references matching the fetch refspec which have disappeared on the
> remote. This step is not currently covered by the `--atomic` flag: we
> delete branches even though updating of local references has failed,
> which means that the fetch is not an all-or-nothing operation.

> Fix this bug by passing in the global transaction into `prune_refs()`:
> if one is given, then we'll only queue up deletions and not commit them
> right away.
>
> This change also improves performance when pruning many branches in a
> repository with a big packed-refs file: every references is pruned in
> its own transaction, which means that we potentially have to rewrite
> the packed-refs files for every single reference we're about to prune.

> The following benchmark demonstrates this: it performs a pruning fetch
> from a repository with a single reference into a repository with 100k
> references, which causes us to prune all but one reference. This is of
> course a very artificial setup, but serves to demonstrate the impact of
> only having to write the packed-refs file once:
>
>     Benchmark 1: git fetch --prune --atomic +refs/*:refs/* (HEAD~)
>       Time (mean ± σ):      2.366 s ±  0.021 s    [User: 0.858 s, System: 1.508 s]
>       Range (min … max):    2.328 s …  2.407 s    10 runs
>
>     Benchmark 2: git fetch --prune --atomic +refs/*:refs/* (HEAD)
>       Time (mean ± σ):      1.369 s ±  0.017 s    [User: 0.715 s, System: 0.641 s]
>       Range (min … max):    1.346 s …  1.400 s    10 runs
>
>     Summary
>       'git fetch --prune --atomic +refs/*:refs/* (HEAD)' ran
>         1.73 ± 0.03 times faster than 'git fetch --prune --atomic +refs/*:refs/* (HEAD~)'