fetch: fix '--force' for pathnames

`brew fetch --force` was a no-op for formula pathnames when that formula
did not exist in Library/Formula, because `brew --cache #{f.name}`
results in an error for that case.

Fix it by always using f.cached_download.

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

Library/Homebrew/cmd/fetch.rb

@@ -20,8 +20,8 @@ module Homebrew extend self
 
     bucket.each do |f|
       if ARGV.include? "--force" or ARGV.include? "-f"
-        where_to = `brew --cache #{f.name}`.strip
-        FileUtils.rm_rf where_to unless where_to.empty?
+        where_to = f.cached_download
+        FileUtils.rm_rf where_to if File.exist? where_to
       end
 
       the_tarball = f.downloader.fetch