Remove LinkedKegs entry if we're writing the same one

If a keg has been uninstalled via `rm -rf <keg>`, and a user tries to
reinstall it without `brew unlink`ing it first, it will fail to link as
the LinkedKegs entry still exists. This isn't desirable, and the user
should be able to reinstall the same formula on top of the old, dead
symlinks without problems, so let's just remove the LinkedKegs entry if
it matches the one we are installing anyway.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Library/Homebrew/cmd/install.rb

@@ -104,6 +104,7 @@ module Homebrew extend self
           fi = FormulaInstaller.new(f)
           fi.install
           fi.caveats
+          f.linked_keg.unlink if f.linked_keg.directory? and f.linked_keg.realpath == f.prefix
           fi.finish
         rescue FormulaAlreadyInstalledError => e
           opoo e.message