fetch: Stop writing FETCH_HEAD
When fetching into a repository, git-fetch(1) by default writes the FETCH_HEAD file containing all references which have been updated. While this isn't much of a problem in "normal-sized" repositories, the file may easily be hundreds of megabytes in size when mirror-fetching from repositories with millions of references. Given that we ultimately don't use that file anywhere this is a big waste of resources.
In "www-gitlab-com", the FETCH_HEAD file is about 379MB in size after performing a mirror-fetch against another replica. The following shows the difference with and without writing FETCH_HEAD:
Benchmark 1: git fetch replica refs/*:refs/*
Time (mean ± σ): 28.292 s ± 0.122 s [User: 25.561 s, System: 3.787 s]
Range (min … max): 28.159 s … 28.398 s 3 runs
Benchmark 2: git fetch --no-write-fetch-head replica refs/*:refs/*
Time (mean ± σ): 27.820 s ± 0.064 s [User: 25.068 s, System: 3.650 s]
Range (min … max): 27.758 s … 27.885 s 3 runs
Summary
'git fetch --no-write-fetch-head replica refs/*:refs/*' ran
1.02 ± 0.00 times faster than 'git fetch replica refs/*:refs/*'While the speed improvement is surprisingly small, it is still the right thing to do.
Changelog: changed