Advanced rebase options

[rarkins]

Today we support:

• Rebase if stale (out of date) by any commit
• Rebase if conflicted

I'm thinking of some others which would be useful:

• Rebase only if versions of other dependencies within the package file changed (e.g. due to a Renovate PR merge)
• Rebase only if the list of dependencies within the package file changed (e.g. users adds or deletes a dependency within the same package.json)

In neither case would it be "repository-aware" of other package files.

The idea of these new ones is to reduce the amount of rebasing compared to rebaseStalePrs=true. Instead of rebasing every commit, you could opt into rebasing only if other dependency versions changed, or if the combination of dependencies changed.

We should probably migrate these options into a single enum type option too, e.g. rebaseWhen with values conflicted,dep-names-changed,dep-versions-changed,behind-base-branch. Default could be auto, with same behaviour as today.