Merge branch 'jc/bare'

* jc/bare:
  Disallow working directory commands in a bare repository.
  git-fetch: allow updating the current branch in a bare repository.
  Introduce is_bare_repository() and core.bare configuration variable
  Move initialization of log_all_ref_updates

git-checkout.sh

@@ -3,6 +3,7 @@
 USAGE='[-f] [-b <new_branch>] [-m] [<branch>] [<paths>...]'
 SUBDIRECTORY_OK=Sometimes
 . git-sh-setup
+require_work_tree
 
 old_name=HEAD
 old=$(git-rev-parse --verify $old_name 2>/dev/null)