bottle: set bottle block tap.

Fix Library/Homebrew/language RuboCop warnings

Avoids issues when using `—keep-old`

cask/cli/cleanup_spec: comment out flaky test.

test-bot: don't try to `git clean` any taps.

This has been causing sporadic CI failures.

Also, unify the cleanup code and be more careful and consistent with
untapping.

test-bot: try again to fix homebrew/core untap.

Be more selective with the `git clean` and remove the broken attempt
to autotap if missing (as it'd be slow anyway).

test-bot: --ci-upload can upload from current dir.

Look in the current directory rather than only allowing upload from
Jenkins.

Call subcommands from zsh completion

bottle: fix old_value/value ordering for --merge.

Previously this was the wrong way around causing it to not be accepting
enough on e.g. a bump from :any to :any_no_relocation.

bottle: use key/value rather than field.

Consistent naming is good.

Sierra: update diagnostic message, pkg-config, Xcode doc

bottle: further improve --keep-old mismatching.

- Make the code more similar between `bottle` and `bottle --merge`.
- Also output the mismatched values when using `--merge`.

tests: always set HOMEBREW_DEVELOPER=1.

Merge Cask's gitignore with root one.

bottle: improve --keep-old failure output.

If fields have changed: tell the user what they were before and are
afterwards.

This fixes the ignoring in libgit2 clients that don't read nested
`.gitignore` files.

Unit tests should be run consistently no matter who is running them and
we can stub `ARGV.homebrew_developer?` if we need to change them.

Add language method to `OS::Mac`.

diagnostic: remove duplicate checks.