[create-release-branch] Add command to view changes since previous release for a package in a monorepo #59

If the user runs this command from within a monorepo:

yarn create-release-branch log @metamask/controller-utils

then it should show a list of commits that have taken place between HEAD and the given package's previous release (going off of the Git tags). This command should be a proxy over git log, so the user should be able to pass arbitrary arguments to this command they would usually be able to pass to git log.

Similarly, if the user runs

yarn create-release-branch diff @metamask/controller-utils

then it should show a commits with changes that have taken place between HEAD and the given package's previous release (going off of the Git tags). This command should be a proxy over git diff, so the user should be able to pass arbitrary arguments to this command they would usually be able to pass to git log.

(I realize that it may be strange to pass commands to a tool that supposedly creates a release branch, but I'm considering renaming this tool to indicate that it can be used to manage releases and not just create them. That's a different ticket, though.)