Add branch upstream management methods

Author: tiennou

ObjectiveGit/GTBranch.h

@@ -51,6 +51,7 @@ NS_ASSUME_NONNULL_BEGIN
 @property (nonatomic, readonly, strong) GTRepository *repository;
 @property (nonatomic, readonly, strong) GTReference *reference;
 @property (nonatomic, readonly, getter=isHEAD) BOOL HEAD;
+@property (nonatomic, assign) GTBranch *upstreamBranch;
 
 + (NSString *)localNamePrefix;
 + (NSString *)remoteNamePrefix;

ObjectiveGit/GTBranch.m

@@ -156,6 +156,19 @@ - (BOOL)isHEAD {
 	return (git_branch_is_head(self.reference.git_reference) ? YES : NO);
 }
 
+- (GTBranch *)upstreamBranch {
+	git_reference *git_ref;
+	int gitError = git_branch_upstream(&git_ref, self.reference.git_reference);
+	if (gitError != GIT_OK) return nil;
+
+	GTReference *ref = [[GTReference alloc] initWithGitReference:git_ref repository:self.repository];
+	return [GTBranch branchWithReference:ref];
+}
+
+- (void)setUpstreamBranch:(GTBranch *)branch {
+	git_branch_set_upstream(self.reference.git_reference, (branch ? branch.name.UTF8String : NULL));
+}
+
 - (NSArray *)uniqueCommitsRelativeToBranch:(GTBranch *)otherBranch error:(NSError **)error {
 	GTEnumerator *enumerator = [self.repository enumeratorForUniqueCommitsFromOID:self.OID relativeToOID:otherBranch.OID error:error];
 	return [enumerator allObjectsWithError:error];