The git log is also handy.
first "merge": https://git.kernel.org/pub/scm/git/git.git/commit/?id=33deb6...
first "tag": https://git.kernel.org/pub/scm/git/git.git/commit/?id=bf0c6e...
first "branch": https://git.kernel.org/pub/scm/git/git.git/commit/?id=74b242...
first Linus "branch" commit: https://git.kernel.org/pub/scm/git/git.git/commit/?id=e69a19...
https://marc.info/?l=git&m=111377572329534
I don't know about 'git branch', but it looks like 'git merge' wasn't a thing
edit: from searching a bit, it appears that it had branches on June of the launch year, dunno if it had those on release.