Remove `isAGitDirectory:`
Remove `isAGitDirectory:`

As discussed in https://github.com/libgit2/objective-git/issues/524 it
is not public and `[GTRepository initWithURL:error:]` should be more
future safe.

libgit2/objective-git,dleehr/objective-git,phatblat/objective-git,dleehr/objective-git,alehed/objective-git,dleehr/objective-git,0x4a616e/objective-git,tiennou/objective-git,nerdishbynature/objective-git,tiennou/objective-git,blackpixel/objective-git,libgit2/objective-git,alehed/objective-git,slavikus/objective-git,0x4a616e/objective-git,slavikus/objective-git,phatblat/objective-git,blackpixel/objective-git,alehed/objective-git,nerdishbynature/objective-git,libgit2/objective-git,phatblat/objective-git,blackpixel/objective-git,nerdishbynature/objective-git,0x4a616e/objective-git,tiennou/objective-git,dleehr/objective-git,slavikus/objective-git,libgit2/objective-git,blackpixel/objective-git