Disallow working directory commands in a bare repository.

If the user tries to run a porcelainish command which requires
a working directory in a bare repository they may get unexpected
results which are difficult to predict and may differ from command
to command.

Instead we should detect that the current repository is a bare
repository and refuse to run the command there, as there is no
working directory associated with it.

[jc: updated Shawn's original somewhat -- bugs are mine.]

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

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)