cmd_diff(): make it obvious which cases are exclusive of each other

cmd_diff(): make it obvious which cases are exclusive of each other

At first glance the OBJ_COMMIT, OBJ_TREE, and OBJ_BLOB cases look like
they might be mutually exclusive.  But the OBJ_COMMIT case doesn't end
the loop iteration with "continue" like the other two cases, but
rather falls through.  So use if...else if...else construct to make it
more obvious that only the last two cases are mutually exclusive.

Signed-off-by: Michael Haggerty <redacted>
Signed-off-by: Junio C Hamano <redacted>