Only use a single parser for tree objects

Only use a single parser for tree objects

This makes read_tree_recursive and read_tree take a struct tree
instead of a buffer. It also move the declaration of read_tree into
tree.h (where struct tree is defined), and updates ls-tree and
diff-index (the only places that presently use read_tree*()) to use
the new versions.

Signed-off-by: Daniel Barkalow <barkalow@iabervon.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>