The commit message doesn't encode the timeline in neither git nor svn, and no-one complains.

So why do you expect the git hash to encode the timeline?

Git encodes the timeline in the parent/child relationship of commits (and/or the date stamps). Trying to look for the timeline information in the hashes is going to be futile, that's not where it's supposed to be found.

For documents that get send back and forth via email, amongst more than 2 participants, git's DAG is actually a better representation than a linear timeline.

(But yes, if you don't like that, you can set up git to reject commits with more than one parent to enforce a linear history.)