devel/go-git: Update 5.10.0 => 5.19.2, take maintainership

While here, sort Makefile (pet portclippy).

Upstream moved the CLI into a nested Go module (cli/go-git has its own
go.mod), which excludes it from the parent module zip. GO_MODULE can no
longer deliver it at any version, which is why the port has been stuck
at 5.10.0.

Switch to fetching sources from GitHub by tag and supply
vendor/modules.txt, which GH_TUPLE does not generate.

Commit log:
https://github.com/go-git/go-git/compare/v5.10.0...v5.19.2

PR: 298162
Reported by: Aydar Gareev <gareev@aydar.me> (takes maintainership)
Approved by: osa, vvd (Mentors, implicit)