We set `core.fsmonitor` to `false` in #13586 because Git's fsmonitor
daemon prevented the release of our locks, resulting in errors for some
users reported in #13521.
However, the fix from #13586 set `core.fsmonitor` to `false` too often.
This is not needed for taps, or when `HOMEBREW_REPOSITORY` is different
from `HOMEBREW_PREFIX`. The issue in #13521 stems from the fsmonitor
daemon preventing the release of our locks, so there is no need to
prevent the daemon from monitoring repositories that don't contain our
lock files.
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