fix(git-branch): read a jj repo's detached HEAD as no branch

[eric-engberg]

Relates to #517: it undoes the part of that issue's problem that showing detached commits (#679) added. Git Changes, which the issue is about, is unchanged; #499 covers that.

What

In a colocated jj repo, Git Branch goes back to what it showed before #679: ⎇ no git, or nothing with "hide no git". Any other git repo's detached HEAD (rebase, bisect, tag checkout) still shows its commit, ⎇ (7d897a5). That includes a git repo nested inside a jj workspace, such as a submodule of a colocated repo or a clone under a jj-managed home directory.

Why

jj keeps git's HEAD detached in a colocated repo, on the working copy's parent. Since #679, Git Branch shows a detached HEAD's commit, so it now shows one in every jj repo: ⎇ (4227868). That commit is jj's bookkeeping, not something the user checked out. #517's reporter pairs the Git widgets with the jj widgets and hides the Git ones outside git repos. Git Branch used to hide in jj repos for them, and since #679 it no longer does.

Checked with jj 0.46.0: after jj git init --colocate and a commit, git symbolic-ref HEAD fails ("ref HEAD is not a symbolic ref"), and git rev-parse --short HEAD gives the commit the widget then printed.

How

On a detached HEAD, GitBranch.render checks that the repo is a colocated jj repo before looking up the commit. Two things must hold: jj root succeeds (isInsideJjRepo, the same check the jj widgets use), and git's top-level directory (git rev-parse --show-toplevel) holds a .jj directory. jj root alone isn't enough. jj finds its workspace by walking up to the nearest .jj and doesn't stop at a git repo's root, so it also succeeds in a git repo nested anywhere under a jj workspace, whose HEAD jj doesn't manage. In a colocated repo the ref is null, which takes the existing "no git" path.

The checks only run on a detached HEAD, so a checked-out branch costs nothing more, and the top-level lookup only runs once jj root has succeeded. Outside a jj workspace, or without jj installed, jj root fails and the commit shows as before. A second workspace from jj workspace add gets its own git worktree with a .jj at its top level, so it reads as no branch like the first. isInsideJjRepo itself is unchanged, so the jj widgets aren't affected.

Demo

Piped renders in a colocated jj repo (bookmark demo), then in a plain git repo on a detached HEAD. Each has Git Branch as is on line 1, and with "hide no git" on line 2, next to Jj Bookmarks.

Powerline

Before (main)	After

Plain

Before (main)	After

Update

These are piped renders against fake scratch repos built with jj 0.46.0, each on a detached HEAD: a colocated jj repo, a git submodule of that repo, and a plain git repo under a jj workspace that isn't colocated. In each group, line 1 shows Git Branch as is and line 2 shows it with "hide no git". Before this fix, both nested repos show "⎇ no git" or hide the widget. After it, they show their commits (973d5d1, 1642305), and the colocated repo still shows no branch, as before.

Powerline, before and after:

Plain, before and after:

Testing

• GitBranch.test.ts:

  • A colocated jj repo's detached HEAD renders ⎇ no git, or nothing with "hide no git", after exactly four commands: the work-tree probe, git symbolic-ref HEAD, jj root and git rev-parse --show-toplevel (no commit lookup). The test points them at a temp directory with a real .jj in it.
  • A git repo nested in that jj workspace (jj root gives the workspace, the top level is code/app inside it) still renders its detached commit, ⎇ (ddfb2de), with "hide no git" on.
  • The two detached-HEAD tests from fix(git): detached HEAD, tag-shadowed branches, submodule worktrees and wide PR titles #679 now answer the jj root call with "no jj repo" and still get the commit and its link.
  • The empty-lookups test gets an answer for the new call.

• bun test: 3115 pass, 0 fail. bun run lint passes. Under Node (Vitest), GitBranch.test.ts passes 29 of 29.

• Built CLI with real jj 0.46.0, piped renders on a detached HEAD, Git Branch as is / with "hide no git". Bun 1.4.2 and Node 26.10.0 print the same in every case. "First commit" is this PR's first commit, which checked jj root alone.

  Repo	First commit	Now
  Colocated jj repo (jj git init --colocate, then a commit)	⎇ no git / hidden	⎇ no git / hidden
  Second workspace (jj workspace add)	⎇ no git / hidden	⎇ no git / hidden
  Git submodule of a colocated repo	⎇ no git / hidden	⎇ (87ae600) / ⎇ (87ae600)
  Plain git repo nested in a non-colocated jj workspace	⎇ no git / hidden	⎇ (8baf8cc) / ⎇ (8baf8cc)
  Plain git repo, no jj above	⎇ (8baf8cc) / ⎇ (8baf8cc)	⎇ (8baf8cc) / ⎇ (8baf8cc)

• Baseline configs (plain and Powerline, example payload), built CLI: identical to main under both runtimes, and the TUI opens and exits under both.