Git-machete

- added: `--ignore-if-missing` flag to `git machete github retarget-pr` command
- added: `--branch=<branch>` option to `git machete github retarget-pr` command
- fixed: `github anno-prs` no longer assumes that local branch and its remote counterpart share the same name
- fixed: `git machete --help` displays a man page (instead of crashing with `No manual entry for git-machete`) when git-machete is installed via Homebrew

- fixed: a few glitches in the animated gif in README

- fixed: interactive rebase triggered by `traverse`, `update` etc. no longer fails when an effectively-empty commit (commit whose changes have already been applied in the given rebase) is encountered

- fixed: `advance` crashing when the current branch is untracked

- deprecated: `machete.overrideForkPoint.<branch>.whileDescendantOf` is no longer taken into account; it's still written, however, for compatibility reasons

- fixed: zsh shell completion for the `slide-out` command no longer fails
- fixed: GitHub token retrieval logic