commit-tree: allow generic object name for the tree as well.

commit-tree: allow generic object name for the tree as well.

We use get_sha1() for -p (parent) objects, but still used
get_sha1_hex() for the tree.  Just to be consistent, allow
extended SHA1 expression for the tree object name.

Note that this is not to encourage funky things like this:

	git-commit-tree HEAD^{tree} -p HEAD

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