[CoreyKaylor]

Adds native Jujutsu support (#11). In a jj workspace, review, staging, commit, log, compare and conflict resolution all run through jj, and diffbandit can act as jj's merge editor and diff editor.

This is open for jj users to try before it merges. Feedback on how the staging model feels is especially welcome.

Commits

1. External merge and diff tool mode. Adds diffbandit merge|edit|diff, a pluggable merge host, and an editable folder diff. It doesn't depend on jj and also works as a Git mergetool.
2. VCS backend proxy. Routes repository calls to Git or jj by repo. No behaviour change on its own, and it fixes three small Git bugs along the way.
3. jj backend. Queues, the squash-workflow staging model, commit and amend, native conflicts, and 50_jj specs run against real jj repos. CI installs jj.
4. Docs. README and :help diffbandit-jj.

Trying it

Point your plugin manager at this branch, for example with lazy.nvim:

{ "CoreyKaylor/diffbandit.nvim", branch = "jj" }

Then run diffbandit (or :DiffBanditLaunch) in a jj repo. To use it as jj's editor too, add this to ~/.config/jj/config.toml, with bin/diffbandit on your PATH:

[merge-tools.diffbandit]
program = "diffbandit"
merge-args = ["merge", "$base", "$left", "$right", "$output", "--path", "$path"]
edit-args = ["edit", "$left", "$right"]
diff-args = ["diff", "$left", "$right"]
merge-tool-edits-conflict-markers = false

[ui]
merge-editor = "diffbandit"
diff-editor = "diffbandit"

How staging maps to jj

• The unstaged view shows @-→@, the same as plain jj diff.
• Staging a hunk or file moves it from @ into the staging change. The staging change is @-, as long as it has no description and can still be edited.
• If @- is already a described commit, your first stage creates an empty staging change under @. An existing commit is never changed by accident.
• Amend mode stages straight into a described @-.
• :w in the panel describes the staging change and opens a fresh one under @.

Worth trying

• jj split, jj squash -i and jj resolve with the config above.
• Staging hunks, committing, and amending from the panel.
• Opening the log and a bookmark compare.
• A colocated repo, including the vcs.prefer = "git" override.

Known limits

• Conflicts with more than two sides, or involving a deletion, are handed off to jj resolve.
• A merge @ lists only its conflicts.
• The log shows commit ids rather than change ids.

Testing

• The unit suite, including the 50_jj specs, and tests/integration/run.sh pass on each commit.
• I checked these by hand in a terminal: jj resolve (quitting keeps the conflict, saving resolves it), jj split, and staging from the panel.

Closes #11