This reverts commit 3e4547f5, reversing
changes made to 6edf9382.

This reverts commit 23728729, reversing
changes made to 3e4547f5.

If people have `HOMEBREW_RUBY_PATH` set then things explode in a rather
confusing fashion. Instead, run `bundle` for them with the arguments
that they'd want.

Also, move `macho` requires into the module itself; it's a pain having
to do everything for Bundler before requiring `pathname` which is a core
Ruby class.

Homebrew's actually ended up using a fair few gems. While we want to
avoid Bundler at runtime (and this PR still does that, in fact uses
Bundler even less at runtime than it did before) writing our own version
to use at build-time seems redundant.

Fix those that can be done so without tearing Homebrew to pieces and
remove the comments for those that can never be done.

Rather than Homebrew/brew or Homebrew/homebrew-core.

It's not necessary to make this a hard failure so don't (and this makes
it more consistent with `brew.sh`).

Fixes #1462.

Fixes https://github.com/Homebrew/homebrew-test-bot/issues/3

For tagged commits produces the output:
- `1.0.1`

For untagged commits with a dirty tree produces the output:
- `1.0.1-19-g23efbc5-dirty`

Performance:
```
git describe --tags --dirty 2> /dev/null
0.07s user 0.01s system 96% cpu 0.086 total
```

This means we can tag any commit without needing to manually remember
to bump the revision every time.

Move the `LinkedKegs` migration into `utils.rb` so it can also be called
from `brew.rb` on startup.

Print a better exception message and handle this in more cases rather
than producing confusing errors when it’s unset.

We'll keep the Homebrew installer pointing to Tigerbrew for now but as
Homebrew/brew technically has no reason to not work on 10.5 let's remove
this check.

Closes #577.

Signed-off-by: Baptiste Fontaine <b@ptistefontaine.fr>

Move some stuff formerly in `Library/ENV` around:
- Move `Library/ENV/$XCODE_VERSION` to `Library/Homebrew/env/super` as they are
  all superenv wrappers and all symlinks to the same version. We never needed
  the "separate shims for separate versions" functionality and it just adds
  confusion.
- Move `Library/ENV/pkgconfig` to `Library/Homebrew/env/pkgconfig` to get more
  things under `Library/Homebrew`
- Move `Library/ENV/scm` to `Library/scm` as these wrappers are not actually
  used by or related to superenv (or stdenv) in any way.

This reverts commit dba1958b.

This reverts commit 252c701c.

Taps installed prior to running the test suite are not visible to the
test suite as most Homebrew paths are redefined as to not mess up the
local installation.

All of these taps use Homebrew internal APIs (or will shortly) and we
autoinstall them all from `brew $CMD`. We should adjust our CI to ensure
that we never accidentally break these taps when making changes to core
code so that these taps can rely more on this core code rather than
having to e.g. vendor equivalent code that never changes on our end.

If the environment variable HOMEBREW_TEST_GENERIC_OS is set ensure that
neither Mac nor Linux-specific code is loaded. This allows easier
testing of cross-platform code on OS X and will make it easier to port
Homebrew to platforms other than OS X and Linux.

Global namespaces are good to avoid when possible.

This version is never wanted at this point and it will help Homebrew
Cask deal with the annoying errors that result from having this version
still around (some which I've already help users debug).

Use the `reason` attribute to be able to handle `UsageError` subclasses
more uniformly and simplify logic in `brew.rb` to handle them together.

Instead of always printing the generic help text, print command-specific
help if it is available and a command raised the `UsageError` exception.
Put the error message underneath the help text (was above) to avoid that
it scrolls off the screen.

Thereby fix a regression where handling the invalid usage would fail to
access `ARGV.usage` removed in c6536066.