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

Otherwise if your `HOMEBREW_PREFIX` and `HOMEBREW_REPOSITORY` are not
equal then your tap manpages will be linked but your `brew*` ones will
not.

If you're using e.g. a `/usr/local/homebrew` prefix then don't require
the `/usr/local/Cellar` to be manually created to avoid e.g.
`/usr/local/homebrew/Cellar` being used. Let's do all we can to let
people use this `Cellar` location as it means they can put their
repository wherever they like and still use all our bottles.

The experiment to add `-no_weak_imports` unconditionally has been a bit
of a failure. It's broken more than it's fixed and I don't think we can
have this as a default for Xcode 8.

Add `ENV.no_weak_imports` to be used by formulae authors and make
`ENV.no_weak_imports a no-op (for now).

We may reconsider this behaviour in future.

Otherwise it can end up as e.g. `bin/git` which then breaks when we
`cd` to another directory and try to run it.

Go makes extensive use of weak imports so we need to allow them when
building Go-using software.

Closes https://github.com/Homebrew/homebrew-core/issues/4047.

Also, check in this manpage, adjust the Rakefile and remove the old one.

Fixes #731.

Flips HOMEBREW_RUBY_MACHO to HOMEBREW_NO_RUBY_MACHO.

After branching, bumping, pushing, and pr-ing; return to whatever branch
was originally checked out.

In most cases, I'd imagine users to want to continue receiving tap
updates from master. However, after using bump-formula-pr, the tap in
which the formula was bumped is left on the working branch that was
doing the bumping and pull-request. After opening the PR, we should
return to whatever branch the user originally had checked out – most
likely master. (But git allows us to just say "previous branch" by using
`-`)

Without `--no-track`, some git setups may automatically set
`origin/master` as the tracked upstream for the newly created branch.
This upstream is what hub defaults as --head when opening PRs. By not
allowing git to set `origin/master` as upstream, hub can then use the
proper --head for the PR.

Since hub still needs to know what --head is intended to be, we can set
the branch's upstream when pushing: with the --set-upstream option.

Fixes #755

These are intentionally duplicated for now until the Cask migration is
complete.