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.

[anthonyryan1]

There are multiple packages which get bumped straight to stable. The current idea is to add a flag to metadata.xml to indicate that a package is expected to be bumped straight to stable, once that gets implemented. This pull request needs to be amended to respect that flag.

For the moment, we are blocked awaiting that new feature.