[feature request] save and auto-load head commit hash for incremental updating of CHANGELOG

[GerHobbelt]

Use case + process:

• you create CHANGELOG via changelog all + hand-editing at time/date T1.
  • changelog has saved to top commit hash in .changelog.json or some such 'hidden' file.
• some time later (time T2) you rerun changelog to UPDATE/APPEND the CHANGELOG.
  • changelog loads the hash from .changelog.json and uses that as the start of the commit range to request commit log info about and generate changelog entries for, which are appended at the top of the CHANGELOG.

Result:

With the above process you can update the CHANGELOG without worrying about missing entries and/or having to wade through duplicates, also when you do this several times when a new release is not yet scheduled, i.e. during development for the next release.

It also saves you the hassle (and very much potential for human error) of coming up with a manually specified commit range to load in changelog: this way you will be sure that changelog will have processed all your commits to date, over time, while your CHANGELOG slowly grows and stays up to date, also between releases.