fixtures: add test.m4a

doctor: remove outdated Homebrew check.

ARGV: kegs: error on empty arguments

test.m4a contains an AAC LC stream inside an M4A (MPEG-4 Part 14)
container. It is generated by FFmpeg 3.1.3 from test.wav:

    ffmpeg -y -i test.wav -c:a aac -profile:a aac_low -b:a 32k test.m4a

test.m4a facilitates testing utilities or libraries that work with the
AAC codec or the M4A container, e.g., alac.

update.sh: don't restore branch to stable.

This is no longer relevant in the bright, new tagged future.

Fixes #1151.

Only run for Homebrew developers so assume they don’t want to switch
back to an old stable branch.

Fixes #1141.

update.sh: use HOMEBREW_UPDATE_TO_TAG.

This makes the code easier to follow rather than having to know
HOMEBREW_NO_UPDATE_CLEANUP implies not updating to a tag.

Fix `search_tap` regex/string matching.

Fixes #1138.

download_strategy: never verbosely untar

The verbose mode of tar is too pointlessly verbose.

RuboCop: Style/CaseEquality

Alter mkdir wrapper use mkdir_p to create intermediate directories

update.sh: use GitHub tags API.

Otherwise if we've committed to `master` and someone `brew update`s
before we cut the tag then they won't be updated to the latest version.

RuboCop: Style/AccessorMethodName

Update manpage.

Add some missed changes. I'll add a CI check for this at a future point.

test-bot: move to tap.

version: don't let a/b imply alpha/beta.

update.sh: always use stable branch for tags.

Migrate to https://github.com/Homebrew/homebrew-test-bot to be able to
work on this with a different, non-tagged workflow.

brew.sh: change directory for HOMEBREW_VERSION.

This is assumption is broken for at least OpenSSL which makes it a
bad general rule.

As discussed in #1102.

This avoids creating a new branch that’ll never be deleted for each tag
and differentiates between the `master` and `stable` branches.

update.sh: do not restore stable (tag) branch

Cask: Use nested classes and modules.

rubocop: allow hash rockets in taps.

Because `depends_on foo: :bar` looks rubbish.

RuboCop: Fix Style/GuardClause.

Fix RuboCop for Taps.

This stops `-dirty` showing up whenever you aren't in
`HOMEBREW_REPOSITORY`.

Fixes #1108.