If we have a `brew.sh` which has set
`HOMEBREW_ENABLE_AUTO_UPDATE_MIGRATION` then let's allow an auto-update
migration. That's because it contains the fix below it _before_ the
update happened which means the auto-update won't fail in the same way
as if updating from an old version.

Set the `PATH` to always have the `bin` and `sbin` for the current
`HOMEBREW_PREFIX` at the beginning.

Fixes #930

Save the value of GEM_HOME before unsetting it,
and resetting it when setting up a gem environment.

Fixes #944.

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.