submodule: do not pass null OID to setup_revisions

If "git pull --recurse-submodules --rebase" is invoked when the current
branch and its corresponding remote-tracking branch have no merge base,
a "bad object" fatal error occurs. This issue was introduced with commit
a6d7eb2 ("pull: optionally rebase submodules (remote submodule
changes only)", 2017-06-23), which also introduced this feature.

This is because cmd_pull() in builtin/pull.c thus invokes
submodule_touches_in_range() with a null OID as the first parameter.
Ensure that this case works, and document what happens in this case.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Reviewed-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: jonathantanmy

submodule.c

@@ -1166,8 +1166,10 @@ int submodule_touches_in_range(struct object_id *excl_oid,
 
 	argv_array_push(&args, "--"); /* args[0] program name */
 	argv_array_push(&args, oid_to_hex(incl_oid));
-	argv_array_push(&args, "--not");
-	argv_array_push(&args, oid_to_hex(excl_oid));
+	if (!is_null_oid(excl_oid)) {
+		argv_array_push(&args, "--not");
+		argv_array_push(&args, oid_to_hex(excl_oid));
+	}
 
 	collect_changed_submodules(&subs, &args);
 	ret = subs.nr;

submodule.h

@@ -94,7 +94,10 @@ extern int merge_submodule(struct object_id *result, const char *path,
 			   const struct object_id *a,
 			   const struct object_id *b, int search);
 
-/* Checks if there are submodule changes in a..b. */
+/*
+ * Checks if there are submodule changes in a..b. If a is the null OID,
+ * checks b and all its ancestors instead.
+ */
 extern int submodule_touches_in_range(struct object_id *a,
 				      struct object_id *b);
 extern int find_unpushed_submodules(struct oid_array *commits,

t/t5572-pull-submodule.sh

@@ -132,4 +132,25 @@ test_expect_success 'pull rebase recursing fails with conflicts' '
 	test_i18ngrep "locally recorded submodule modifications" err
 '
 
+test_expect_success 'branch has no merge base with remote-tracking counterpart' '
+	rm -rf parent child &&
+
+	test_create_repo a-submodule &&
+	test_commit -C a-submodule foo &&
+
+	test_create_repo parent &&
+	git -C parent submodule add "$(pwd)/a-submodule" &&
+	git -C parent commit -m foo &&
+
+	git clone parent child &&
+
+	# Reset master so that it has no merge base with
+	# refs/remotes/origin/master.
+	OTHER=$(git -C child commit-tree -m bar \
+		$(git -C child rev-parse HEAD^{tree})) &&
+	git -C child reset --hard "$OTHER" &&
+
+	git -C child pull --recurse-submodules --rebase
+'
+
 test_done