handle_revision_arg: simplify commit reference lookups

The "dotdot" range parser avoids calling
lookup_commit_reference() if we are directly fed two
commits. But its casts are unnecessarily complex; that
function will just return a commit we pass into it.

Just calling the function all the time is much simpler, and
doesn't do any significant extra work (the object is already
parsed, and deref_tag() on a non-tag is a noop; we do incur
one extra lookup_object() call, but that's fairly trivial).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

revision.c

@@ -1500,12 +1500,8 @@ int handle_revision_arg(const char *arg_, struct rev_info *revs, int flags, unsi
 				struct commit *a, *b;
 				struct commit_list *exclude;
 
-				a = (a_obj->type == OBJ_COMMIT
+				a = lookup_commit_reference(a_obj->oid.hash);
-				     ? (struct commit *)a_obj
+				b = lookup_commit_reference(b_obj->oid.hash);
-				     : lookup_commit_reference(a_obj->oid.hash));
-				b = (b_obj->type == OBJ_COMMIT
-				     ? (struct commit *)b_obj
-				     : lookup_commit_reference(b_obj->oid.hash));
 				if (!a || !b)
 					goto missing;
 				exclude = get_merge_bases(a, b);