The flag GIT_FILEBUF_FORCE currently does two things:
     1. It will cause the filebuf to create non-existing leading
        directories for the file that is about to be written.
     2. It will forcibly remove any pre-existing locks.
While most call sites actually do want (1), they do not want to
remove pre-existing locks, as that renders the locking mechanisms
effectively useless.
Introduce a new flag `GIT_FILEBUF_CREATE_LEADING_DIRS` to
separate both behaviours cleanly from each other and convert
callers to use it instead of `GIT_FILEBUF_FORCE` to have them
honor locked files correctly.

As this conversion removes all current users of `GIT_FILEBUF_FORCE`,
this commit removes the flag altogether.