Make useGitCommitTimestamp use commit timestamp instead of author timestamp

## Issue

**version**: 1.9.0
**usage context**:
- [x] maven command line: 3.8.3

**Problem description**:

One reason for adding a timestamp to the version is that you want to have strictly increasing versions, where you can easily see which of two given versions is the newer one.

Currently, `useGitCommitTimestamp` is using the author timestamp of the commit. This is the timestamp that tells you when the author created the commit.
Then there is the commit timestamp, which is the timestamp that tells you when the commit was pushed.

Using the author timestamp as a version is a problem, because you do not gete strictly increasing versions. If I merge a PR that I have been working on for a couple of months, the version will look like it is two months old. But in fact it is the latest version, merged just a few seconds ago.