Git-branchstack

- BREAKING: option `--trim-subject` has been dropped and is the default
behavior. New option `--keep-tags` restores the old behavior. Consequently,
the meaning of `+` inside topic tags has been inverted.
- Preserve committer date in created commits. This means we create
deterministic commit IDs, which makes it easier to reuse commits, and
creates fewer loose objects.

- BREAKING `git-branchless` was renamed to `git-branchstack` (1).
- `git-branchstack-pick` replaces a "refs/" prefix with "gitref/", making it
easier to cherry-pick remote non-branch refs without introducing ambiguous
refs.

- `git-branchless-pick` now prefers `$GIT_SEQUENCE_EDITOR` over `git var $EDITOR`
for editing the rebase-todo list.
- `git-branchless-pick` now supports empty base commits, so `..some-branch`
means: pick all commits on `some-branch` minus the commits already in `HEAD`.
- `git-branchless-pick ..some/branch` will no longer trim the `some/` prefix,
unless `some` is a valid Git remote.

- First release on PyPI.
- Specify dependencies with a `+` prefix, like `[child:+parent]`, to include
commits from `parent` and trim their subject tags.
- `git-branchless-pick` no longer pulls in new commits from `{upstream}`.
- Fix subject computation for conflict hint commits without topic prefix

- BREAKING: `git-branchless-pick` takes a `..`-range instead of a single commit.
- Branches are no longer based on {upstream} but on `git merge-base {u} HEAD`
- Similarly, `git-branchless-pick` will only not add new commits from {upstream}
- Support multline subjects

- BREAKING: the latest version of git-revise is now required, see README
- On conflict, show commits that are likely missing as dependencies
- Allow passing a custom range with -r to override {upstream}..HEAD
- Allow dropping topic tags from subject with -t/--trim-subject
- Fixed a case of mistakenly refusing to overwrite branches after
cancelling a previous run (usually on conflict)
- git-branchless-pick inserts new commits in the rebase-todo list
immediately after dropped commits, instead of before them