Added -updateTrackingBranch:error:.

Author: joshaber

ObjectiveGit/GTBranch.h

@@ -79,6 +79,15 @@ typedef NS_ENUM(NSInteger, GTBranchType) {
 /// found, returns nil and sets `success` to YES.
 - (GTBranch *)trackingBranchWithError:(NSError **)error success:(BOOL *)success;
 
+/// Update the tracking branch.
+///
+/// trackingBranch - The tracking branch for the receiver. If nil, it unsets the
+///                  tracking branch.
+/// error          - The error if one occurred.
+///
+/// Returns whether it was successful.
+- (BOOL)updateTrackingBranch:(GTBranch *)trackingBranch error:(NSError **)error;
+
 /// Reloads the branch's reference and creates a new branch based off that newly
 /// loaded reference.
 ///

ObjectiveGit/GTBranch.m

@@ -205,6 +205,16 @@ - (GTBranch *)trackingBranchWithError:(NSError **)error success:(BOOL *)success
 	return [[self class] branchWithReference:[[GTReference alloc] initWithGitReference:trackingRef repository:self.repository] repository:self.repository];
 }
 
+- (BOOL)updateTrackingBranch:(GTBranch *)trackingBranch error:(NSError **)error {
+	int result = git_branch_set_upstream(self.reference.git_reference, trackingBranch.name.UTF8String);
+	if (result != GIT_OK) {
+		if (error != NULL) *error = [NSError git_errorFor:result description:@"Failed to update tracking branch for %@", self];
+		return NO;
+	}
+
+	return YES;
+}
+
 - (GTBranch *)reloadedBranchWithError:(NSError **)error {
 	GTReference *reloadedRef = [self.reference reloadedReferenceWithError:error];
 	if (reloadedRef == nil) return nil;