Don't Repeat Yourself.

- Don't enforce hash rockets on external commands.
- Enabled accidentally disabled ParenthesesAsGroupedExpression.

Another look at the current Rubocop rules and how they fit with our
existing and desired future style. Almost all of these changes were
automatic. Split some rules between formulae/brew where brew doesn't
have millions of cases that need fixed.

Outputs the merged pull requests on Homebrew/brew between two Git refs.

New name and old name must have different names

* Causes a bug in Formula#installed_alias_target_changed? when
  Formula#superseds_an_installed_formula? returns true because
  Formula#old_installed_formulae includes f for some Formula f.

* Causes a bug when foo@2.4 with alias foo has HEAD or devel version and
  we try to `brew upgrade foo --devel|--HEAD` from stable. The upgrade fails
  while since we installing formula to the same prefix it's alredy installed.
  The reason for that is that we use
  `formula_to_install = outdated.map(&:latest_formula)` in cmd/upgrade
  before calling upgrade_formula on foo.

  ```ruby
  def latest_formula
  installed_alias_target_changed? ? current_installed_alias_target : self
  end
  ```

  Formula#installed_alias_target_changed? compares formulae using
  Formula#==, which is wrong for this case, thus Formula#latest_formula doesn't
  return self and returns Formula#current_installed_alias_target with spec
  foo was initially installed instead of devel or HEAD, causing the error.

Suppose you have devel and stable versions of `foo` installed.
Their versions should be grouped together regardless their specs.

Output before the change:
  foo (2.4), foo (3.28-01) < 5.1

Output after the change:
  foo (2.4, 3.28-01) < 5.1

fixes 2 issues: error when username has a number, correct url for repos not on github

Small improvement to the xcode license hint. Reduces friction by allowing user to accept the license in a single step. 
Reference: http://apple.stackexchange.com/questions/175069/how-to-accept-xcode-license

Handle the case when an argument passed to `brew cask` is both the name
of a Cask and a non-Cask file in the current working directory.

Fixes https://github.com/caskroom/homebrew-cask/issues/25611.

This is handled by `brew style` now.

This avoids needlessly installing build-time dependencies of dependencies
which are not themselves being built from source.

- unset IFS to fix version parsing on older Bashes
- reuse existing HOMEBREW_MACOS_VERSION if it’s available
- set rather than unset popup_stub

On older versions of OS X (i.e. pre-Mavericks) `xcode-select` is not
required to be set when using the CLT. Fix this behaviour so `brew
update` doesn't tell everyone on that configuration to `brew install
git`.