fix multi-stack commits

[Byron]

As follow-up to #7952, this PR is another attempt at fixing it.
Whereas #7952 applies a fix as post-process, let's try again and see if this can be correct from the start.

Tasks

• reproduction with additional debugging
  • From what we found out in our session with @krlvi, the integration commit as passed to the commit-engine already only has a single parent. If it had not, it would loudly fail with an error
    as the base_substitute must be set.
• add test (with respective virtual-branches state)
• fix it by detecting the case that not all heads are yet represented in the worktree commit, and rewrite the workspace commit to have it, then use the rebase engine as normal which does a proper merge.
  • make sure that failures don't actually make any change (probably automatic as no ref-rewrites would happen then)

• double-check situation in the UI works as well, without the fix.

The Problem

To reproduce the bug (with the previous fix unapplied), one has to be in a state where a new branch has been created, and now we attempt to create a new commit that new branch.

2025-04-04 18:05 +0800 GitButler         o [gitbutler/workspace] GitButler Workspace Commit
2025-04-04 18:04 +0800 Sebastian Thiel   o [dependent] [refs/gitbutler/Lane-4] left branch change
2025-03-26 14:25 +0800 Sebastian Thiel   o selection
2025-03-25 16:24 +0800 Sebastian Thiel   o some data for file
2025-03-25 10:01 +0800 Sebastian Thiel   o binary
2025-03-12 09:55 +0800 Sebastian Thiel   o tar added
2025-03-12 09:55 +0800 Sebastian Thiel   o edited in UI
2025-03-12 09:55 +0800 Sebastian Thiel   o [below-dependent] {origin/below-dependent} top commit
2025-02-25 20:31 +0100 Sebastian Thiel   o below middle below top
2025-02-27 19:56 +0100 Sebastian Thiel   o at A
2025-02-27 19:56 +0100 Sebastian Thiel   o below A
2025-02-27 19:56 +0100 Sebastian Thiel   o 2
2025-02-27 19:56 +0100 Sebastian Thiel   o 1
2025-02-24 18:53 +0100 Sebastian Thiel   o [new-branch] {origin/master} nested file

The current implementation isn't able to deal with that as it expect any head that is part of the worktree to already be merged into the worktree commit.

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