Re: [PATCH 1/6] fetch: speed up lookup of want refs via commit-graph

> When updating our local refs based on the refs fetched from the remote,
> we need to iterate through all requested refs and load their respective
> commits such that we can determine whether they need to be appended to
> FETCH_HEAD or not. In cases where we're fetching from a remote with
> exceedingly many refs, resolving these refs can be quite expensive given
> that we repeatedly need to unpack object headers for each of the
> referenced objects.
> 
> Speed this up by opportunistcally trying to resolve object IDs via the
> commit graph: more likely than not, they're going to be a commit anyway,
> and this lets us avoid having to unpack object headers completely in
> case the object is a commit that is part of the commit-graph. This
> significantly speeds up mirror-fetches in a real-world repository with
> 2.3M refs:
> 
>     Benchmark #1: HEAD~: git-fetch
>       Time (mean ± σ):     56.942 s ±  0.449 s    [User: 53.360 s, System: 5.356 s]
>       Range (min … max):   56.372 s … 57.533 s    5 runs
> 
>     Benchmark #2: HEAD: git-fetch
>       Time (mean ± σ):     33.657 s ±  0.167 s    [User: 30.302 s, System: 5.181 s]
>       Range (min … max):   33.454 s … 33.844 s    5 runs
> 
>     Summary
>       'HEAD: git-fetch' ran
>         1.69 ± 0.02 times faster than 'HEAD~: git-fetch'

> -			commit = lookup_commit_reference_gently(the_repository,
> -								&rm->old_oid,
> -								1);
> -			if (!commit)
> -				rm->fetch_head_status = FETCH_HEAD_NOT_FOR_MERGE;
> +			commit = lookup_commit_in_graph(the_repository, &rm->old_oid);
> +			if (!commit) {
> +				commit = lookup_commit_reference_gently(the_repository,
> +									&rm->old_oid,
> +									1);
> +				if (!commit)
> +					rm->fetch_head_status = FETCH_HEAD_NOT_FOR_MERGE;