Upgrade to Rubocop 0.57.2

Remove unneeded RuboCop comment.

DependencyOrder: Fix var initialization bug

Fix `Cask#full_name`.

Readd functionality to list all casks.

linkage: enable cache by default.

brew.sh: remove unused variable.

As of #4377 this is no longer used.

portable-ruby: respect HOMEBREW_BOTTLE_DOMAIN variable.

This has not been causing any issues in CI or for `master` users so
let's now enable it by default for everyone.

This was requested in:
https://discourse.brew.sh/t/adding-additional-download-sources-to-homebrew/2446

With this change you can use e.g. Artifactory for Homebrew's portable
ruby.

Implement HOMEBREW_FORCE_BREWED_GIT

No need to check for both variables, because they're always set together.
Also, this harmonizes with the CURL equivalent just above.

Because of this messing with the user's path:
https://github.com/Homebrew/brew/blob/
    efc02899/bin/brew#L68
brew uses /usr/bin/git over brewed git, even when the former is
problematically old.
There may also be other reasons a user prefers to use brewed git.

There was already a HOMEBREW_FORCE_BREWED_CURL option and a
HOMEBREW_SYSTEM_CURL_TOO_OLD check to set it. This mostly copies those
to implement HOMEBREW_FORCE_BREWED_GIT & HOMEBREW_SYSTEM_GIT_TOO_OLD.

See also: https://github.com/Linuxbrew/brew/issues/736

Revert "Fix SVN downloads of versioned formula by munging directory names"

mirror: use curl_executable instead of curl function

mirror: invoke correct curl

Remove remaining use of `system "curl"` in favor of the `curl` function.

Fix SVN downloads of versioned formula by munging directory names

[WIP] formula_installer: Don't check devtools if pouring

cache_store: create HOMEBREW_CACHE when needed.

Fixes #4366.

cask fetch and upgrade: print caveats

Update `search` docs.

zsh completion: Remove redundant exclusion lists for single options