Adjusting SVN commands to consider the @ symbol

formula: handle unavailable versioned formulae.

If people decide to copy these into their Homebrew/homebrew-core
`Formula` directory then it breaks generating the hash for any
non-versioned formulae.

See https://github.com/Homebrew/homebrew-bundle/issues/351

Revert "Use `SystemCommand` for `curl`."

Raise error when file does not contain a cask.

Use `SystemCommand` for `curl`.

audit: change the "unstable" spec message to devel/head

Don’t match `MZ` for `Cab`.

link: when refusing to link display keg only caveats instead.

These are a bit easier to follow and have been recently improved.

Inspired by conversation in #4441.

- make it a public method
- allow skipping the reason
- output how to set the various variables in your current shell

Let’s have a default value for shell (considering this isn’t a public
API) to make it easier to use.

Tweak code coverage logic

- make OS detection looser to exclude more Linux files on macOS (and
  vice versa)
- Allow slight (0.5%) coverage drops to account for somewhat random
  fluctuations.

Make `HOMEBREW_TAP_PATH_REGEX` also match exact path.

Add `Tap#command_file?` method.

Don’t rescue `NameError` for `#run`.

Remove `MachO` check for `Executable`.