checkout -f failed to check out a file if an existing directory interfered.

checkout -f failed to check out a file if an existing directory interfered.

When path foo/bar existed in the working tree, checkout -f to switch to
a branch that has a file foo silently did a wrong thing.  It failed to
remove the directory foo, did not check out the file foo, and the worst
of all it did not report any errors.

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