Do not require clean tree when reverting and cherry-picking.

Do not require clean tree when reverting and cherry-picking.

My stupidity deserved to be yelled at by Linus ... there is no reason
to require the working tree to be clean when merging -- the only
requirements are index to match HEAD commit and the paths involved in
merge are up to date in the working tree.  Revert and cherry-pick are
just specialized forms of merge, and the requirements should be the
same.

Remove the 'general purpose routine to make sure tree is clean' from
git-sh-setup, to prevent me from getting tempted again.

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