Snitch purge may fail with empty commit

[rexim]

Steps to reproduce

1. Add the following line to any file in a repo:

// TODO(#<already closed issue id>): some text

2. Don't commit the change. Leave the tree in the dirty state.
3. snitch purge
4. Confirm to delete the recently added TODO

Observed

The snitch removed the TODO but failed to commit the change because there no change. By removing the TODO snitch cleaned up the tree.

Expected

Snitch ignores the empty commit error (maybe issues a warning) and continues deleting the rest of the schedule TODOs.