[[feature-stackdir]] Spec: Optional SHA-1s; property flags.

Some externally-stored properties are expected to be regularly
modified concurrently by multiple users -- for example, scripts.
Storing a SHA-1 of the file means that a merge conflict will occur for
every concurrent modification and will have negative workflow
consequences (e.g. it will prevent the use of pull requests on
Github).  They should therefore be optional.

It is useful to store optional flag bits along with property
descriptors.  These have a number of applications.