Optimize CoreTap in `.deleted_reason`
Optimize CoreTap in `.deleted_reason`

This limits the previous change to only the
core tap in `MissingFormula.deleted_reason`.

The reason for this is that the `git log` command
only becomes a performance issue when the number
of monthly commits is high which really only happens
with the core tap.

Homebrew/brew,Homebrew/brew,Homebrew/brew,Homebrew/brew