Make path in checkoutProgressBlock optional

The path from the checkout progress block is set to nil when
called the first time from git_checkout_iterator. This happens
when calling report_progress() on line 2547 in checkout.c. This
in turn causes a crash.

Author: Alexander Hedges

ObjectiveGit/GTRepository.h

@@ -228,7 +228,7 @@ extern NSString * const GTRepositoryInitOptionsOriginURLString;
 ///                         May be NULL.
 ///
 /// returns nil (and fills the error parameter) if an error occurred, or a GTRepository object if successful.
-+ (nullable instancetype)cloneFromURL:(NSURL *)originURL toWorkingDirectory:(NSURL *)workdirURL options:(nullable NSDictionary *)options error:(NSError **)error transferProgressBlock:(nullable void (^)(const git_transfer_progress *, BOOL *stop))transferProgressBlock checkoutProgressBlock:(nullable void (^) (NSString *path, NSUInteger completedSteps, NSUInteger totalSteps))checkoutProgressBlock;
++ (nullable instancetype)cloneFromURL:(NSURL *)originURL toWorkingDirectory:(NSURL *)workdirURL options:(nullable NSDictionary *)options error:(NSError **)error transferProgressBlock:(nullable void (^)(const git_transfer_progress *, BOOL *stop))transferProgressBlock checkoutProgressBlock:(nullable void (^) (NSString *__nullable path, NSUInteger completedSteps, NSUInteger totalSteps))checkoutProgressBlock;
 
 /// Lookup objects in the repo by oid or sha1
 - (nullable id)lookUpObjectByOID:(GTOID *)oid objectType:(GTObjectType)type error:(NSError **)error;

ObjectiveGit/GTRepository.m

@@ -223,7 +223,7 @@ static int remoteCreate(git_remote **remote, git_repository *repo, const char *n
 	return GIT_OK;
 }
 
-+ (instancetype)cloneFromURL:(NSURL *)originURL toWorkingDirectory:(NSURL *)workdirURL options:(NSDictionary *)options error:(NSError **)error transferProgressBlock:(void (^)(const git_transfer_progress *, BOOL *stop))transferProgressBlock checkoutProgressBlock:(void (^)(NSString *path, NSUInteger completedSteps, NSUInteger totalSteps))checkoutProgressBlock {
++ (instancetype)cloneFromURL:(NSURL *)originURL toWorkingDirectory:(NSURL *)workdirURL options:(NSDictionary *)options error:(NSError **)error transferProgressBlock:(void (^)(const git_transfer_progress *, BOOL *stop))transferProgressBlock checkoutProgressBlock:(void (^)(NSString *__nullable path, NSUInteger completedSteps, NSUInteger totalSteps))checkoutProgressBlock {
 
 	git_clone_options cloneOptions = GIT_CLONE_OPTIONS_INIT;