Git-machete

- added: `git machete completion bash|fish|zsh` command
- fixed: multiple glitches in the existing bash/fish/zsh completions

- improved: layout of documentation at ReadTheDocs
- improved: replaced `definition file` with `branch layout file` across the docs

- fixed: building the package for Ubuntu PPA

- fixed: `fish` completion no longer prompts file names alongside commands/flags (contributed by guyboltonking)

- fixed: `git machete github` not being able to retrieve token used by `gh` for `gh` version >= 2.31.0 (reported by domesticsimian)

- fixed: `machete-post-slide-out`, `machete-pre-rebase` and `machete-status-branch` hooks can now be executed on Windows
- fixed: unstable behavior after `edit` option has been selected for interactively sliding out invalid branches
- fixed: handling of HTTP redirects when `git machete github create-pr` and `retarget-pr` act on a repository that has been renamed and/or moved
- improved: `git machete github retarget-pr` now fails if there are multiple PR with the given head branch (rather than silently take the first of them into account)