See https://github.com/caskroom/homebrew-cask/issues/24519

Feel free to suggest alternative wording and such.

On auto-update `HOMEBREW_LIBRARY` may change location which means that
it won't be found for the actual install command. Look for this having
occurred and then set the new `HOMEBREW_LIBRARY` (and
`HOMEBREW_REPOSITORY`) accordingly.

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).

This will make brew readall --syntax pass

Previously, this method would:

- Include the current formula
- Include the current target of the installed alias, which by definition
  is not "old"

This commit fixes both of these issues.
Only formulae that are the current target of the alias they were installed
with are now considered to have old installed formulae.

`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.

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.

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).