Add a function to make a directory and intervening directories, honoring
core.sharedRepository.
To do so, add a utility function to perform an operation with the umask
set to a certain value. This is required because os.MkdirAll honors the
umask and is potentially recursive (so we can't fix things after
creating them). Without temporarily setting the umask, it isn't possible
to create directories with the right permissions if those permissions
are looser than the original umask.
