Expand and enhance git-last-modified(1) documentation

Option -z and --max-depth are not documented in git-last-modified(1) while they are pretty crucial. In these patches documentation is added in the man page and the -h output.

Issue: #600

--- b4-submit-tracking ---

This section is used internally by b4 prep for tracking purposes.

{ "series": { "revision": 1, "change-id": "20251114-toon-last-modified-zzzz-af9c1be74fc4", "prefixes": [] } }