Fix brew update local changes bug

Users were seeing local changes in their repository after updating, even
though they had made no local changes.

The repository setup sequence should use `git reset --soft` rather than
vanilla `git reset`, which defaults to '--mixed'. '--soft' updates
_only_ HEAD, leaving the index as-is, allowing future incantations of
`brew update` to proceed without errors.

Fixes Homebrew/homebrew#6732.

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 FETCH_HEAD"
+          safe_system "git reset --soft FETCH_HEAD"
         rescue Exception
           safe_system "rm -rf .git"
           raise