Allow Multiselect Cherry-pick using Interactive Rebase

Now that SourceTree has an interactive rebase dialog, the next natural step is to allow us to multi-select several commits from a second branch and "cherry-pick" them onto the current branch.

This can also be viewed as a rebase operation where a number of the commits from the second branch are already marked to be "skipped." But in general, rebase and cherry-pick should be viewed from a technical perspective as interactive rebase operations. For example, and because it makes for popular word-games at Git Tech parties:

• "rebase" cherry-picks all the commits from one branch onto another.
• "cherry-pick" rebases a single commit from one branch onto another.