Reset --hard after the first git init

We don't know what revision the tarball was when homebrew was installed, so all we can do really is hard reset. In theory some set of git commands should suffice, but apparently --soft and --mixed resets ended up putting stuff in the index which would break subsequent `brew updates`.

Fixes Homebrew/homebrew#6851

Library/Homebrew/cmd/update.rb

@@ -42,7 +42,7 @@ class RefreshBrew
         begin
           safe_system "git init"
           safe_system "git fetch #{REPOSITORY_URL}"
-          safe_system "git reset --soft FETCH_HEAD"
+          safe_system "git reset --hard FETCH_HEAD"
         rescue Exception
           safe_system "rm -rf .git"
           raise