This updates the git-flow-avh plugin to the latest version
from https://github.com/petervanderdoes/git-flow-completion
(0.6.0 at the time of this commit).

This mainly adds support for the "git flow bugfix" command.

This PR replaces #4626 as well as #4705, both of which adds support for the bugfix command but doesn't use the official version from https://github.com/petervanderdoes/git-flow-completion

Fixes #4705
Fixes #4626
Fixes #5903