New port: devel/git-absorb: Git command for automating fixup/autosquash commits

You have a feature branch with a few commits. Your teammate reviewed the branch
and pointed out a few bugs. You have fixes for the bugs, but you don't want to
shove them all into an opaque commit that says fixes, because you believe in
atomic commits. Instead of manually finding commit SHAs for git commit --fixup,
or running a manual interactive rebase, do this:

git add $FILES_YOU_FIXED
git absorb
git rebase -i --autosquash master

git absorb will automatically identify which commits are safe to modify, and
which indexed changes belong to each of those commits. It will then write
fixup! commits for each of those changes. You can check its output manually if
you don't trust it, and then fold the fixups into your feature branch with
git's built-in autosquash functionality.

PR: 235436
Submitted by: Greg V <greg@unrelenting.technology>