bin/commit: Add '=' to --statoverride

bin/commit: Add '=' to --statoverride

Previously, we only supported additions in the statoverride file;
it was mainly for adding the setuid bit without having that physically
on disk.

However, for testing a change to `bare-user` handling around *unreadable*
files (which happens for `/etc/shadow` in host content), I need a way
to write that into a repo in the test suite.

I'm not actually aware of a non-test-suite use case for this; a more
sophisticated user is going to be using the API directly, which can already do
this. But we need it for tests at least.

Closes: #989
Approved by: jlebon