gitDirectoryURL can never be nil

It's asserted upon in libgit2.

Author: tiennou

ObjectiveGit/GTRepository.h

@@ -156,7 +156,7 @@ typedef NS_ENUM(NSInteger, GTRepositoryStateType) {
 /// Returns nil for a bare repository.
 @property (nonatomic, readonly, strong) NSURL * _Nullable fileURL;
 /// The file URL for the repository's .git directory.
-@property (nonatomic, readonly, strong) NSURL * _Nullable gitDirectoryURL;
+@property (nonatomic, readonly, strong) NSURL *gitDirectoryURL;
 
 /// Is this a bare repository (one without a working directory)?
 @property (nonatomic, readonly, getter = isBare) BOOL bare;

ObjectiveGit/GTRepository.m

@@ -648,7 +648,7 @@ - (NSURL *)fileURL {
 
 - (NSURL *)gitDirectoryURL {
 	const char *cPath = git_repository_path(self.git_repository);
-	if (cPath == NULL) return nil;
+	NSAssert(cPath, @"gitdirectory is nil");
 
 	NSString *path = @(cPath);
 	NSAssert(path, @"gitdirectory is nil");