git-checkout: handle local changes sanely when detaching HEAD
git-checkout: handle local changes sanely when detaching HEAD

When switching branches, we usually first try read-tree to make
sure that we do not lose the local changes and then updated the
HEAD using update-ref.  However, we detached and updated HEAD
before these checks, which was quite bad in a repository with
local changes.

Signed-off-by: Junio C Hamano <dc50d1021234060e53ec42a77d526afa2fe07479@cox.net>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git