core: Add generic "commit filter" API, use it to implement statoverride

core: Add generic "commit filter" API, use it to implement statoverride

The builder wants the ability to mark a given file as e.g. setuid.  To
implement this, the repo now has a callback-based API when importing a
directory to modify or remove items.

The commit tool accepts a "statoverride" file as input which looks like:

+mode /path/to/file