checkout: sometimes work from a subdirectory.

checkout: sometimes work from a subdirectory.

git-checkout does two very different things, and what they
should do when run from subdirectory are quite different.

It does not make any sense to run the one that switches the
current head from anywhere other than the toplevel:

	git-checkout [-f] <branch>
        git-checkout [-b <branch>] <committish>

We could of course chdir to top and do the whole-tree checkout
in git-checkout, but the point is the operation does not make
sense on a partial tree.  The whole tree is checked out.

The other form is to update the index file and working tree file
selectively:

	git-checkout <treeish> <file>... ;# out of tree to index and file
        git-checkout -- <file>...	 ;# out of index to file

This form _does_ make sense to run from subdirectory; and I
myself often wish we supported this.

So here is a patch to do both.

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