Add ability to cancel the remote part of the clone operation

This makes the transferProgressBlock look more like its libgit equivalent
the git_transfer_progress_cb.

Author: Alexander Hedges

ObjectiveGit/GTRepository.h

@@ -206,11 +206,12 @@ extern NSString * const GTRepositoryInitOptionsOriginURLString;
 ///                         `GTRepositoryCloneOptionsServerCertificateURL`
 /// error                 - A pointer to fill in case of trouble.
 /// transferProgressBlock - This block is called with network transfer updates.
+///                         A negative return value cancels the operation.
 /// checkoutProgressBlock - This block is called with checkout updates
 ///                         (if `GTRepositoryCloneOptionsCheckout` is YES).
 ///
 /// returns nil (and fills the error parameter) if an error occurred, or a GTRepository object if successful.
-+ (id)cloneFromURL:(NSURL *)originURL toWorkingDirectory:(NSURL *)workdirURL options:(NSDictionary *)options error:(NSError **)error transferProgressBlock:(void (^)(const git_transfer_progress *))transferProgressBlock checkoutProgressBlock:(void (^)(NSString *path, NSUInteger completedSteps, NSUInteger totalSteps))checkoutProgressBlock;
++ (id)cloneFromURL:(NSURL *)originURL toWorkingDirectory:(NSURL *)workdirURL options:(NSDictionary *)options error:(NSError **)error transferProgressBlock:(int (^)(const git_transfer_progress *))transferProgressBlock checkoutProgressBlock:(void (^)(NSString *path, NSUInteger completedSteps, NSUInteger totalSteps))checkoutProgressBlock;
 
 /// Lookup objects in the repo by oid or sha1
 - (id)lookUpObjectByOID:(GTOID *)oid objectType:(GTObjectType)type error:(NSError **)error;

ObjectiveGit/GTRepository.m

@@ -184,7 +184,7 @@ - (id)initWithURL:(NSURL *)localFileURL error:(NSError **)error {
 }
 
 
-typedef void(^GTTransferProgressBlock)(const git_transfer_progress *progress);
+typedef int(^GTTransferProgressBlock)(const git_transfer_progress *progress);
 
 static void checkoutProgressCallback(const char *path, size_t completedSteps, size_t totalSteps, void *payload) {
 	if (payload == NULL) return;
@@ -197,8 +197,7 @@ static int transferProgressCallback(const git_transfer_progress *progress, void
 	if (payload == NULL) return 0;
 	struct GTClonePayload *pld = payload;
 	if (pld->transferProgressBlock == NULL) return 0;
-	pld->transferProgressBlock(progress);
-	return 0;
+	return pld->transferProgressBlock(progress);
 }
 
 struct GTClonePayload {
@@ -222,7 +221,7 @@ static int remoteCreate(git_remote **remote, git_repository *repo, const char *n
 	git_remote_callbacks remoteCallbacks;
 };
 
-+ (id)cloneFromURL:(NSURL *)originURL toWorkingDirectory:(NSURL *)workdirURL options:(NSDictionary *)options error:(NSError **)error transferProgressBlock:(void (^)(const git_transfer_progress *))transferProgressBlock checkoutProgressBlock:(void (^)(NSString *path, NSUInteger completedSteps, NSUInteger totalSteps))checkoutProgressBlock {
++ (id)cloneFromURL:(NSURL *)originURL toWorkingDirectory:(NSURL *)workdirURL options:(NSDictionary *)options error:(NSError **)error transferProgressBlock:(int (^)(const git_transfer_progress *))transferProgressBlock checkoutProgressBlock:(void (^)(NSString *path, NSUInteger completedSteps, NSUInteger totalSteps))checkoutProgressBlock {
 
 	git_clone_options cloneOptions = GIT_CLONE_OPTIONS_INIT;