Fix exception handling for opening bitmap index files:
When creating a new PackFile instance it may have an associated bitmap index file or not. This information is cached and the JGit assumed a bitmap index file must exist if the cached data tells so. But it may happen that the packfiles are repacked during a gc in a different process causing the packfile, bitmap-index and index file to be deleted. Since JGit still has an open file handle on the packfile this file is still accessible for this JGit proces until the file handle is closed. But index and bitmap index file are deleted and can't be accessed for other processes. Fix getBitmapIndex() to invalidate the cached packfile instance if such a situation occurs.