Tweak HOMEBREW_DEVELOPER documentation.

diagnostic: don't barf if HEAD is also a file.

Enable sandbox by default for homebrew/core

This option does more than just print warnings so let's make a more
generic description with a more specific example.

Closes #716.

- Inconsistent or unneeded indentation
- Missing or superfluous empty lines
- Missing or wrongly formatted arguments in command summary
- Missing punctuation

Closes #694.

tests: add cmd/audit unit tests

list: note that `ls` command is used.

Enable vendored Ruby 2.0.

Add a new `Sandbox.formula?` method to see if a given formula should be
sandboxed. Use the formula to check its tap against a list of
pre-approved taps where we know every formula builds under the sandbox
(currently just homebrew/core).

Simplify checking if we’re going to sandbox a test with `Sandbox.test?`.

brew: sanitize BASH_ENV

Test that sandbox does not complain about bogus .pyc errors and does
complain about other failures.

Closes #684.

These are never fatal and often confusing.

Fixes #683.

update-test: tweak master not updated check.

Loosen this a bit; we don't necessarily expect the end commit is the one
we're looking for, just that it has changed from the start commit (i.e.
some sort of update has occurred).

Addresses some false negatives on `master` branch merges that weren't
present on the PR commits.

And arguments can be passed through to it.

Tweak produced out of conversation in
https://github.com/Homebrew/brew-evolution/pull/8.

Ship Homebrew auto-update support.

search: remove thread-unsafe Hash cache.

Silence auditing formula revisions deprecations

update: add --force argument.

tests: refactor FormulaTextTests in test_cmd_audit, add assertion to test_simple_valid_formula

install_dependencies: Do not truncate dependencies

pull: non-core tap support for Homebrew Testing jobs

versions should be initialized even if formula is not installed

`test-bot --ci-testing` supports a `--tap` option for non-core taps, so
`brew pull` should too when pulling Homebrew Testing jobs.

Swats away this annoying warning when the test is skipped due to GPG being
unavailable:

```
  1) Skipped:
GpgTest#test_create_test_key:
GPG Unavailable

Error:
GpgTest#test_create_test_key:
NoMethodError: undefined method `rmtree' for nil:NilClass
    /usr/local/Library/Homebrew/test/test_gpg.rb:11:in `teardown'
```

Closes #675.

Signed-off-by: Dominyk Tiller <dominyktiller@gmail.com>

Removes the detection logic from the Requirement in favour of it living inside
the Gpg class & us calling it from there. It's a bit nicer & avoids us calling
Requirement code from outside of direct requirement handling & fulfillment.