preliminary version of rmport using git

[Rene Ladan]

Hi,

just a quick note to say that Tools/scripts/rmport is mostly converted
to git. Some outstanding issues:
- Replace the git merge with an equivalent of "git push freebsd
    working:main" ? The first method feels more familiar/natural to me
  whilst the second one is what [1] prescribes.
- Test the changes on the dev repository.

After that I plan to tackle the other big scripts (mfh and addport)
and some smaller scripts (svn/git-wise) after that.

The current diff is available at [2].

[1] https://github.com/bsdimp/freebsd-git-docs/blob/main/src-cvt.md
[2] https://people.freebsd.org/~rene/stuff/rmport.diff

Regards,
René