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.

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

Fixes #1141.

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

The verbose mode of tar is too pointlessly verbose.

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.

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

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.

Because `depends_on foo: :bar` looks rubbish.

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

Fixes #1108.

Restoring stable branch post-update could lead to unsuspecting users
with homebrew.devcmdrun being stuck forever on an old tag.

Fixes #1111.