Set origin remote URL when updating git clones

Thus the url we use can change.

This only works provided the urls both contain git repos with the same blobs. But this should always be true.

Library/Homebrew/download_strategy.rb

@@ -296,9 +296,7 @@ class GitDownloadStrategy < AbstractDownloadStrategy
   end
 
   def fetch
-    raise "You must install Git:\n\n"+
-          "  brew install git\n" \
-          unless system "/usr/bin/which git"
+    raise "You must install Git: brew install git" unless system "/usr/bin/which -s git"
 
     ohai "Cloning #{@url}"
 
@@ -321,7 +319,8 @@ class GitDownloadStrategy < AbstractDownloadStrategy
     else
       puts "Updating #{@clone}"
       Dir.chdir(@clone) do
-        quiet_safe_system 'git', 'fetch', @url
+        safe_system 'git', 'remote', 'set-url', 'origin', @url
+        quiet_safe_system 'git', 'fetch', 'origin'
         # If we're going to checkout a tag, then we need to fetch new tags too.
         quiet_safe_system 'git', 'fetch', '--tags' if @spec == :tag
       end