Fix first `brew update` with Git 1.7.4+; Fixes Homebrew/homebrew#5128

Behaviour for git checkout was changed such that the update would fail because it refused to checkout files on to existing files in the working directory.

This was bad behaviour anyway, we should make efforts to keep any local modifications to the Homebrew checkout. Everything is neatly resolved if we just do a --soft reset.

Closes Homebrew/homebrew#6017.

Library/Homebrew/cmd/update.rb

@@ -13,7 +13,6 @@ end
 
 class RefreshBrew
   REPOSITORY_URL   = "http://github.com/mxcl/homebrew.git"
-  INIT_COMMAND     = "git init"
   CHECKOUT_COMMAND = "git checkout -q master"
   UPDATE_COMMAND   = "git pull #{REPOSITORY_URL} master"
   REVISION_COMMAND = "git rev-parse HEAD"
@@ -40,7 +39,14 @@ class RefreshBrew
         safe_system CHECKOUT_COMMAND
         @initial_revision = read_revision
       else
-        safe_system INIT_COMMAND
+        begin
+          safe_system "git init"
+          safe_system "git fetch #{REPOSITORY_URL}"
+          safe_system "git reset FETCH_HEAD"
+        rescue Exception
+          safe_system "rm -rf .git"
+          raise
+        end
       end
       execute(UPDATE_COMMAND)
       @current_revision = read_revision