install: unlink old kegs

If a formula makes it all the way to the actual install step, it is safe
to unlink the keg before linking; this will prune dead symlinks in the
case where a keg was removed with `rm -rf <keg>` but not unlinked with
`brew unlink`.

Fixes Homebrew/homebrew#10077.

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

Library/Homebrew/cmd/install.rb

@@ -103,6 +103,10 @@ module Homebrew extend self
         begin
           fi = FormulaInstaller.new(f)
           fi.install
+          # Due to the nature of Keg#unlink, this will remove symlinks from an
+          # older keg, which may still be present if an uninstallation was done
+          # via `rm -rf <keg>`; this is desired.
+          Keg.new("#{f.rack}/#{f.version}").unlink
           fi.caveats
           fi.finish
         rescue FormulaAlreadyInstalledError => e