Add a new pkgdev commit mangle to remove stable keywords

[anthonyryan1]

One of the most common maintenance tasks in the tree is a simple version bump. With this mangle, pkgdev commit can automatically remove the stable keyword from ebuilds copied from a prior version.

This should reduce contributor mistakes, and save developers time on both code review and manually editing keywords for their commits.

[anthonyryan1]

Following some discussion in #gentoo-dev-help

--mangle false allows this to be disabled on a per-commit basis, piggybacking the existing option. @thesamesam expressed some opinion that this should have a separate toggle, but I felt like general helpful mangling all kind of fit well under that existing flag.

One problem I realized with this, is that it could destabilize a package unintentionally if the whole package is moved by git mv

For example git mv old-category/package new-category/package or git mv category/oldname category/newname would be caught up in this, and the new name would lose it's stability flags.

[anthonyryan1]

I've added a second commit to cover the scenario where a package is renamed or moved to another category.

Because git treats all add & delete in the same commit as mv operations, we need to use a different approach to distinguish the two scenarios, we're now only removing stable keywords when the PV changes.

Happy to squash the commits, but I've left them separate until discussion is a bit further along.