Revamp save with improved messages

[mih]

There have been many attempts to get this (more) right. Few pointers:

• Consolidate commit message specification datalad#3896
• ENH(working Draft): with explicitly empty save message, invoke editor to provide one datalad#3893
• ENH: Enable interactive commit message entry datalad#4121

A key challenge has always been the recursive save and how get the message content properly -- the issue being that save needs to run bottom-up, but messages typically make more sense from the top.

With next-status the starting point has changed. It provides a fast overview of the changes and allow for generating nicer hints for users.

There is only one thing missing:

• Add pathspec support to next_status() #598

Once that is done, we can and should redo save.

When we do, it may make sense to also explore message templates. See https://commitizen-tools.github.io/commitizen/ for some inspiration.