When multiple kegs are installed, `brew info` marks a keg with an
asterisk if f.installed_prefix == keg, but this is only true if either
HEAD or the newest version of the formula is installed. This isn't
always useful, so let's mark the currently linked keg with an asterisk
regardless of the version.
Obviously, keg-only formula will never be marked, but this is probably a
feature. When multiple keg-only kegs exist, chances are that they are
each being utilized by something, so there isn't really a "used" and
"unused" version.
Signed-off-by: Jack Nagel <jacknagel@gmail.com>