[PATCH] Rename and extend read_tree_with_tree_or_commit_sha1

This patch renames read_tree_with_tree_or_commit_sha1() to
read_object_with_reference() and extends it to automatically
dereference not just "commit" objects but "tag" objects.  With
this patch, you can say e.g.:

    ls-tree $tag
    read-tree -m $(merge-base $tag $HEAD) $tag $HEAD
    diff-cache $tag
    diff-tree $tag $HEAD

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