octopus-merge (part 1: tree-editing)

Implement an octopus merge based on trees, and (mostly) equivalent to merge-ORT in Git.

[x] tree-editor plumbing
[x] cursor support for editor
[x] prevent writing grocely invalid trees (\0 in filename, should be validate path components?)
[x] benchmark with/without cursor
[x] tree-editor in gix
- [x] proper validation with gix_validate::path::component()
- [x] convenience by default by making it easier to pass relative paths?
- [x] cursor support
- [x] write in-memory

git2-equivalent merge (first version essentially, hoping it's not as complex as a full merge-ORT)

Everything is about MergeORT.

it uses an empty tree if there is no merge-base - we must allow the same.
it allows for multiple merge-bases, creating a virtual one by merging all merge-bases together using the same algorithm, recursively.
merges can have conflicts without a individual files being involved, for instance when directory renames clash
Must make sure that possible types of conflicts are properly communicated, to not degenerate information
It puts conflict-markers in the result tree, with annotations to provide additional context
Need resolution configuration, see git2::MergeOptions.

Byron / gitoxide