Add autocomplete for psykorebase on the git command.

[Natim]

When using:

git psy\t

It should autocomplete to:

git psykorebase

[benoitbryon]

👍

[Natim]

This is something I did here: https://github.com/tj/git-extras/pull/411/files#diff-d5721c14a8e7dadffb2ea455dbe10b9fR112

_git_psykorebase(){
  __gitcomp "$(__git_heads) --continue --no-ff --ask"
}