Git-machete

- added: `-f`/`--as-first-child` flag to `git machete add` (contributed by matthalp)
- fixed: `git machete github retarget-pr` not updating description of PR due to stray `\r` characters

- fixed: make fork-point also take into account common ancestors (and not only reflogs) in more cases

- fixed: if a PR has a pre-v3.23.0 `Based on PR ...` header, then it's removed by `git machete github retarget-pr` in favor of the new extended PR chain

- added: full chain of PRs (and not just a link to the base PR) is added to/updated in PR description by `git machete github create-pr`/`retarget-pr`/`restack-pr` (suggested by mjgigli)
- fixed: in the unlikely case of a cycle between GitHub PRs, `git machete github checkout-pr` aborts with an error rather than falling into an infinite loop
- fixed: when checking out longer PR chains, `git machete github checkout-prs` prints out all checked out branches correctly

- improved: if neither `.git/info/description` nor `.github/pull_request_template.md` is present, `git machete github create-pr` now uses message body of the first unique commit as PR description (suggested by kamaradclimber)
- added: `machete.github.forceDescriptionFromCommitMessage` git config key that forces `git machete github create-pr` to use message body of the first unique commit as PR description (suggested by kamaradclimber)

- fixed: `Cannot parse Link header` error in `git machete github` subcommands when there are more than 100 PRs in the given repository (reported by domesticsimian)
- fixed: if `.git/machete` doesn't exist, `git machete add <branch>` adds both current branch and the newly-added `<branch>` (not just the latter)