Convert lookup_commit* to struct object_id

Convert lookup_commit, lookup_commit_or_die,
lookup_commit_reference, and lookup_commit_reference_gently to take
struct object_id arguments.

Introduce a temporary in parse_object buffer in order to convert this
function.  This is required since in order to convert parse_object and
parse_object_buffer, lookup_commit_reference_gently and
lookup_commit_or_die would need to be converted.  Not introducing a
temporary would therefore require that lookup_commit_or_die take a
struct object_id *, but lookup_commit would take unsigned char *,
leaving a confusing and hard-to-use interface.

parse_object_buffer will lose this temporary in a later patch.

This commit was created with manual changes to commit.c, commit.h, and
object.c, plus the following semantic patch:

@@
expression E1, E2;
@@
- lookup_commit_reference_gently(E1.hash, E2)
+ lookup_commit_reference_gently(&E1, E2)

@@
expression E1, E2;
@@
- lookup_commit_reference_gently(E1->hash, E2)
+ lookup_commit_reference_gently(E1, E2)

@@
expression E1;
@@
- lookup_commit_reference(E1.hash)
+ lookup_commit_reference(&E1)

@@
expression E1;
@@
- lookup_commit_reference(E1->hash)
+ lookup_commit_reference(E1)

@@
expression E1;
@@
- lookup_commit(E1.hash)
+ lookup_commit(&E1)

@@
expression E1;
@@
- lookup_commit(E1->hash)
+ lookup_commit(E1)

@@
expression E1, E2;
@@
- lookup_commit_or_die(E1.hash, E2)
+ lookup_commit_or_die(&E1, E2)

@@
expression E1, E2;
@@
- lookup_commit_or_die(E1->hash, E2)
+ lookup_commit_or_die(E1, E2)

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

builtin/rev-parse.c

@@ -279,8 +279,8 @@ static int try_difference(const char *arg)
 		if (symmetric) {
 			struct commit_list *exclude;
 			struct commit *a, *b;
-			a = lookup_commit_reference(oid.hash);
-			b = lookup_commit_reference(end.hash);
+			a = lookup_commit_reference(&oid);
+			b = lookup_commit_reference(&end);
 			exclude = get_merge_bases(a, b);
 			while (exclude) {
 				struct commit *commit = pop_commit(&exclude);
@@ -332,7 +332,7 @@ static int try_parent_shorthands(const char *arg)
 		return 0;
 	}
 
-	commit = lookup_commit_reference(oid.hash);
+	commit = lookup_commit_reference(&oid);
 	if (exclude_parent &&
 	    exclude_parent > commit_list_count(commit->parents)) {
 		*dotdot = '^';