AddWorktree(path) with no mode silently creates a branch named after the folder (or checks out an existing one); remarks claim a branch is always named

[matt-edmondson]

What's wrong

In GitIntegration/Builders/GitWorktreeAddBuilder.cs, when no mode method and no commit-ish are set, AddMode.Plain emits only git worktree add --end-of-options <path> (L190-L206). Given that command, git does one of three things:

• It creates a new branch named after the last path segment. Running git worktree add --end-of-options ../wt1 printed Preparing worktree (new branch 'wt1').
• If a branch with that name already exists and isn't checked out elsewhere, it checks that branch out.
• With the host setting worktree.guessRemote=true, it can instead base the new branch on a matching remote branch.

The class remarks (L72-L77) justify leaving out --guess-remote with "--guess-remote extends that to the case where no branch is named at all, which this builder always names." That is not true in the Plain/no-commit-ish case.

Failure scenario

A caller who wants "a worktree at ../feature-x" calls repo.AddWorktree(path).ExecuteAsync(). Depending on the folder name and host config, they get:

• an unrequested branch feature-x left in the repository, or
• someone else's existing feature-x branch checked out in the new worktree, which means later commits land on the wrong branch.

The library otherwise pins host config to make results deterministic, but here the result depends on worktree.guessRemote.

Suggested fix

• When neither a mode nor a commit-ish is set, either emit --detach (a worktree at HEAD with no branch side effects), or throw InvalidOperationException asking the caller to choose Branch/NewBranch/Detach/a commit-ish.
• Correct the --guess-remote remark to match.
• Add a builder test for the argument list in the bare AddWorktree(path) case, and an integration test asserting that no new branch appears.

[matt-edmondson]

Triage

• Category: Bug
• Priority: Medium. A bare AddWorktree(path) has side effects the caller didn't ask for: a stray branch, or an existing branch checked out, which can send later commits to the wrong branch. The result also depends on the host's worktree.guessRemote, which breaks the library's determinism guarantee. It doesn't hit callers who pick a mode explicitly.
• Area / suggested assignment: Worktrees: Builders/GitWorktreeAddBuilder (AddMode.Plain, class remarks). @matt-edmondson
• Duplicates: None found. Worktrees() reports the wrong path for a worktree whose path contains a newline, and returns lock/prunable reasons still C-quoted #141 (Worktrees() path and reason parsing) is a different worktree bug.
• In progress: No matching open PR. [patch] Unstage with reset so it works before the first commit #143 changes Unstage only.

Notes: Throwing InvalidOperationException when no mode or commit-ish is given is the more explicit of the two options, and it matches how the other builders require a choice. --detach is the non-breaking option if existing callers depend on a bare call succeeding.

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