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/checkout.c

@@ -393,7 +393,7 @@ static int checkout_paths(const struct checkout_opts *opts,
 		die(_("unable to write new index file"));
 
 	read_ref_full("HEAD", 0, rev.hash, NULL);
-	head = lookup_commit_reference_gently(rev.hash, 1);
+	head = lookup_commit_reference_gently(&rev, 1);
 
 	errs |= post_checkout_hook(head, head, 0);
 	return errs;
@@ -833,7 +833,7 @@ static int switch_branches(const struct checkout_opts *opts,
 	int flag, writeout_error = 0;
 	memset(&old, 0, sizeof(old));
 	old.path = path_to_free = resolve_refdup("HEAD", 0, rev.hash, &flag);
-	old.commit = lookup_commit_reference_gently(rev.hash, 1);
+	old.commit = lookup_commit_reference_gently(&rev, 1);
 	if (!(flag & REF_ISSYMREF))
 		old.path = NULL;
 
@@ -1047,7 +1047,7 @@ static int parse_branchname_arg(int argc, const char **argv,
 	else
 		new->path = NULL; /* not an existing branch */
 
-	new->commit = lookup_commit_reference_gently(rev->hash, 1);
+	new->commit = lookup_commit_reference_gently(rev, 1);
 	if (!new->commit) {
 		/* not a commit */
 		*source_tree = parse_tree_indirect(rev->hash);