Colin Walters e8dbaa3c07 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
2012-01-22 13:27:38 -05:00
..
2011-12-18 17:35:30 -05:00
2011-11-10 13:17:04 -05:00
2011-11-10 13:17:04 -05:00
2012-01-11 14:33:13 -05:00