Make writing the ignore rule more explicit.

Author: joshaber

ObjectiveGit/GTSubmodule.h

@@ -60,9 +60,7 @@ NS_ASSUME_NONNULL_BEGIN
 @property (nonatomic, strong, readonly) GTRepository *parentRepository;
 
 /// The current ignore rule for this submodule.
-///
-/// Setting this property will only update the rule in memory, not on disk.
-@property (nonatomic, assign) GTSubmoduleIgnoreRule ignoreRule;
+@property (nonatomic, readonly, assign) GTSubmoduleIgnoreRule ignoreRule;
 
 /// The OID that the submodule is pinned to in the parent repository's index.
 ///
@@ -111,6 +109,16 @@ NS_ASSUME_NONNULL_BEGIN
 /// Returns whether reloading succeeded.
 - (BOOL)reload:(NSError **)error;
 
+/// Write a new ignore rule to disk and get the resulting submodule. The
+/// receiver will not have the new ignore rule. To update the receiver, call
+/// `-reload:`.
+///
+/// ignoreRule - The ignore rule.
+/// error      - The error if one occurred.
+///
+/// Returns the updated submodule or nil if an error occurred.
+- (GTSubmodule *)submoduleByUpdatingIgnoreRule:(GTSubmoduleIgnoreRule)ignoreRule error:(NSError **)error;
+
 /// Synchronizes the submodule repository's configuration files with the settings
 /// from the parent repository.
 ///

ObjectiveGit/GTSubmodule.m

@@ -25,12 +25,16 @@ - (GTSubmoduleIgnoreRule)ignoreRule {
 	return (GTSubmoduleIgnoreRule)git_submodule_ignore(self.git_submodule);
 }
 
-- (void)setIgnoreRule:(GTSubmoduleIgnoreRule)ignoreRule {
-	git_submodule_set_ignore(self.parentRepository.git_repository, git_submodule_name(self.git_submodule), (git_submodule_ignore_t)ignoreRule);
+- (GTSubmodule *)submoduleByUpdatingIgnoreRule:(GTSubmoduleIgnoreRule)ignoreRule error:(NSError **)error {
+	int result = git_submodule_set_ignore(self.parentRepository.git_repository, git_submodule_name(self.git_submodule), (git_submodule_ignore_t)ignoreRule);
+	if (result != GIT_OK) {
+		if (error != NULL) {
+			*error = [NSError git_errorFor:result description:@"Couldn't set submodule ignore rule."];
+		}
+		return nil;
+	}
 
-	// The docs for `git_submodule_set_ignore` note "This does not affect any
-	// currently-loaded instances." So we need to reload.
-	git_submodule_reload(self.git_submodule, 0);
+	return [self.parentRepository submoduleWithName:self.name error:error];
 }
 
 - (GTOID *)indexOID {