Make "tree_entry" have a SHA1 instead of a union of object pointers

Make "tree_entry" have a SHA1 instead of a union of object pointers

This is preparatory work for further cleanups, where we try to make
tree_entry look more like the more efficient tree-walk descriptor.

Instead of having a union of pointers to blob/tree/objects, this just
makes "struct tree_entry" have the raw SHA1, and makes all the users use
that instead (often that implies adding a "lookup_tree(..)" on the sha1,
but sometimes the user just wanted the SHA1 in the first place, and it
just avoids an unnecessary indirection).

Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>