Otherwise random e.g. `pr-123` tags may cause `brew update` to update
to the wrong version.

Previously, .simplecov called `SimpleCov.result` to store the coverage
result, and ignored the return value. `SimpleCov.result`'s return can be
slow to calculate, which wastes a lot of time when it's ignored.

This commit extracts the code needed to store the SimpleCov result from
`SimpleCov.result`, and calls it directly, without doing the busywork to
compute the return value every time.

In my testing, this more than halves the time taken to run all the
integration tests.

If we have a HOMEBREW_REPOSITORY and HOMEBREW_PREFIX mismatch (now the
default) then we can block access to the whole of HOMEBREW_REPOSITORY
rather than just the HOMEBREW_LIBRARY and `.git`.

This reverts commit a124680b.

Will need to be smarter than this, since people can't force Travis to
update. I'm losing count of the amount of times Travis has forced a change
of plans around Homebrew this year.

Closes https://github.com/Homebrew/brew/issues/1096.

`sha1` isn't a very accurate description of what this value can be
because it can be any git commit reference. For example, `brew test-bot`
uses `brew update-test --commit=HEAD`.