Cherry-pick upstream commit to ensure that completions for commands
like mkfs.xfs(8) end up in mkfs.xfs.fish by fixing over-eager stripping
of file extensions.
All mkfs.*(8) (and fsck.*(8), etc.) programs would yield mkfs.fish files
with unique `complete -c mkfs ...` lines using the same command.
Thus completing `mkfs` in fish would for example yield XFS specific
suggestions, given "xfsprogs" was installed **and** won over other
packages.
Noticed through build logs:
```
building '/nix/store/5fz5jgkd2ycx5fk3lb30my2hr2br639f-system_fish-completions.drv'...
warning: collision between `/nix/store/1z01j2pma0l005w3qg6slndn0rp136s8-dosfstools-4.2_fish-completions/mkfs.fish' and `/nix/store/jxxgrng41l6qakg45fksk1qpil976vgi-xfsprogs-5.16.0_fish-completions/mkfs.fish'
```
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