Previously if I did `brew install boxen/brews/imagemagick` and then `brew
upgrade boxen/brews/imagemagick` and the version of `imagemagick` in
`boxen/brews` was older than the version in `Homebrew/homebrew` then it would
install the correct version from the `boxen/brews` tap and then try to
immediately upgrade it to the version from `Homebrew/homebrew`. I'd argue
fairly strongly that this behaviour is pretty unintuitive; when you fully
specify a formula from a tap then it should be prioritised by `brew upgrade`
and `brew outdated.

This commit makes `brew upgrade boxen/brews/imagemagick` only upgrade the
version of `imagemagick` if the version in the `boxen/brews` tap is newer.
Similarly `brew outdated imagemagick` and `brew outdated
boxen/brews/imagemagick` will show different results if the newer version
differs between `boxen/brews` and `Homebrew/homebrew`.

Closes Homebrew/homebrew#36699.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>