checkout -f: do not leave untracked working tree files.

checkout -f: do not leave untracked working tree files.

Earlier we did not consider untracked working tree files
"precious", but we have always considered them fair game to
clobber.  These days, branch switching by read-tree is more
careful and tries to protect untracked working tree files.  This
caused the following workflow to stop working:

	git checkout one-branch-with-file-F
	git checkout -f another-without-file-F
	git pull . one-branch-with-file-F

Because the second checkout leaves F from the previous state as
untracked file in the working tree, the merge would fail, trying
to protect F from being clobbered.

This changes "git checkout -f" to remove working tree files that
are known to git in the switched-from state but do not exist in
the switched-to state, borrowing the same logic from "reset --hard".

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