This should allow better handling flaky tests.

This should help with cases like
https://github.com/Homebrew/homebrew-bundle/issues/751
and is good practise in general.

Also, document the `brew update --preinstall` flag that is being used
here so others can run it manually e.g. as part of CI if needed.

- make `brew readall` only check Homebrew/brew syntax with no tap
  passed.
- pass specified tap to `brew readall` test for speed
- only shell out once in `FormulaInfo` test for speed

Co-authored-by: Eric Knibbe <enk3@outlook.com>

This is really, really slow at the moment for a few reasons:
- it goes through the list of revisions twice
- it checks many more revisions than it needs to

Even after these improvements it's still by far the slowest audit so
am also making it a `--git` only audit.

Additionally, to further improve default `brew audit` performance do not
run `brew style` checks when doing `brew audit` with no arguments.

`brew style` can be run quickly and efficiently on all of a tap (and is
cached) so no need to duplicate it here.

We no longer use this and `hub checkout` does what we want better.