Closes https://github.com/Homebrew/homebrew-core/issues/4973

References #1023.

Can't see how this could ever happen but let's safe-guard it just in
case (as ilovezfs saw it).

update-report: migrate everyone to new repository.

Use new "macOS" naming where appropriate.

update-report: remove cask dir after migration.

keg: only prune var/homebrew/linked.

Relocate HOMEBREW_REPOSITORY when necessary.

`var/homebrew` is too broad and ends up deleting and recreating the
locks directory repeatedly.

This will hang around otherwise due to e.g. running `bundle install` in
here once.

update-report: cleanup git harder in migration.

Before stashing also try to cancel any current merge, rebase and
reset. This will prevent any data loss but also make the stash more
likely to succeed.

update-report: also create Cellar on migration.

update-report: handle directory not existing.

test-bot: uninstall all formula on cleanup.

Not quite a mass replacement as I've used OS X and Mac OS X where
describing specific older versions and added compatibility methods
for things in the DSL.

Just on the off chance it doesn't exist (i.e. no-one has installed
anything yet or you're a `test-bot` that's run `git clean -xdf`
recently)

Need to do this manually now that we can't rely on `git clean -xdf` to
do it for us.

This shouldn't be possible to occur after #1013 but it makes sense to
add it anyway.

Now that the default from the installer, our CI and soon all users
is `/usr/local/Homebrew` it's a lot easier to check if there's
references to it (as we cannot look for `/usr/local` as it's a too
commonly hardcoded path).

update-report: create top-level directories later.

Otherwise if they are created and empty they may be removed again
before we try to create the `/usr/local/bin/brew` symlink.

Also migrate people without `HOMEBREW_DEVELOPER` set to the new
location.

update-report: migrate /usr/local repository.

update.sh: turn origin/branch into a variable.

Migrate a HOMEBREW_REPOSITORY and HOMEBREW_PREFIX of `/usr/local` so
that the HOMEBREW_REPOSITORY is now in `/usr/local/Homebrew`. This gives
us more flexibility on changing the repository layout in future and
avoids putting junk in `/usr/local` when not necessary (e.g. our
`README.md`).

Rather than repeating origin multiple times.

Rubocop: apply auto-corrections and don't use hash-rockets

utils: convert formula name from pathname to string

formula: set TERM to dumb during test

`name` is passed as argument to Formulary.factory which could later be
downcased at formulary.rb:349, but Pathname doesn't have a downcase
method. Converting `name` to String as it should be fixes the problem.

Fixes #1000.

How-To-PR: both stable & devel in simple commit message

Improve legacy symlink migration.

Read the old symlinks and migrate them across rather than assuming a
unlink/link will work fine (as users have conflicts.)

ENV.no_weak_imports should be a no-op when Xcode doesn't support the
feature to avoid breaking builds with <= Xcode 7.