versions: work for tapped formulae

brew versions is hardcoded to cd to HOMEBREW_REPOSITORY before running
git, and as such fails to report previous versions for any formulae
from a tapped repository.

Add two new private methods repository and entry_name to replace the
hardcoded HOMEBREW_REPOSITORY and formula path, and brew versions
work for both builtin and tapped formulae.

Closes #12356.

Reported-by: Misty De Meo <mistydemeo@gmail.com>
Suggested-by: Jack Nagel <jacknagel@gmail.com>
Signed-off-by: Jiang Xin <worldhello.net@gmail.com>