Created attachment 213452 [details]
Python 2 and Asciidoctor are both deprecated and scheduled to be removed from ports at the end of the year -- in addition, upstream has supported the asciidoctor gem as a documentation build tool for some time now. As such, we can switch over to asciidoctor instead in ports.
Currently however, contrib/subtree does not build with asciidoctor 2.x. As such, backport a patch from upstream  to enable building with newer versions of asciidoctor. This patch can/should be dropped once upstream includes the aforementioned patch in the next release.
(In reply to Bjorn Neergaard from comment #0)
There is an update to textproc/asciidoc to use the Python3 port of the project waiting for review here bug #245723. Would that be worth considering?
I do think this is the better route to go -- upstream is moving to asciidoctor as their build tool, instead of asciidoc3, and asciidoc3 installs as a2x3, instead of a2x. As such we would still have to patch git (or have the asciidoctor port install a symlink). If we go this route, once the upstream patch is merged in the next version, no patches/FreeBSD-specific behavior is needed.
Yes, then I agree that asciidoctor indeed makes more sense for git.
A commit references this bug:
Date: Fri May 8 12:32:45 UTC 2020
New revision: 534399
devel/git: Use rubygem-asciidoctor to build docs
asciidoctor and python2 are deprecated and upstream is moving to
asciidoctor gem as default tool to build documentation
Submitted by: Bjorn Neergaard <email@example.com>
Sponsored by: Rubicon Communications, LLC (Netgate)