builtin/diff-tree: convert to struct object_id

builtin/diff-tree: convert to struct object_id

Convert most leaf functions to struct object_id.  Change several
hardcoded numbers to uses of parse_oid_hex.  In doing so, verify that we
when we want two trees, we have exactly two trees.

Finally, in stdin_diff_commit, avoid accessing the byte after the NUL.
This will be a NUL as well, since the first NUL was a newline we
overwrote.  However, with parse_oid_hex, we no longer need to increment
the pointer directly, and can simply increment it as part of our check
for the space character.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>