Git 2.24 introduced a feature flag for repositories with many files, see:
https://github.blog/2019-11-03-highlights-from-git-2-24/#feature-macros
Since Spack's Git repository contains roughly 8,500 files, it can be
worthwhile to enable this, especially on slow file systems such as NFS:
```
$ hyperfine --warmup 3 'cd spack-default; git status' 'cd spack-manyfiles; git status'
Benchmark #1: cd spack-default; git status
Time (mean ± σ): 3.388 s ± 0.095 s [User: 256.2 ms, System: 625.8 ms]
Range (min … max): 3.168 s … 3.535 s 10 runs
Benchmark #2: cd spack-manyfiles; git status
Time (mean ± σ): 168.7 ms ± 10.9 ms [User: 98.6 ms, System: 126.1 ms]
Range (min … max): 144.8 ms … 188.0 ms 19 runs
Summary
'cd spack-manyfiles; git status' ran
20.09 ± 1.42 times faster than 'cd spack-default; git status'
```
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