Fix "cannot detect diffs" error

[MaggieShan]

Description

• This fixes an issue where the code review view would occasionally flash the "cannot detect diffs" error despite being in a git repo with valid diffs to render
• The bug is that a local diff state model defaults to DiffState::NotInRepository on creation which the view maps to this error message
  • This used to work when the view also treated NotInRepository as a loading state if repo_path.is_some(), but with the refactors made to support a remote diff state model, the semantics for NotInRepository changed to be a valid error where the path was definitively not inside a git repo
• The fix introduces a new starting state where DiffState::Detecting explicitly means that we're actively detecting the repo before loading the diffs

Linked Issue

Reported in:

• Code Review panel briefly shows repository error when switching repos #11914
• https://warpdev.slack.com/archives/C08KTPNQN65/p1780093159997789
• https://warpdev.slack.com/archives/C08KTPNQN65/p1780349380531869

Testing

https://www.loom.com/share/7bab61e0fe624f4d8a5cdfc440e28258

• I have manually tested my changes locally with ./script/run

Agent Mode

• Warp Agent Mode - This PR was created via Warp's AI Agent Mode

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@MaggieShan

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[oz-for-oss]

Overview

This PR introduces an explicit Detecting internal state for the local diff model, maps that in-progress state to DiffState::Loading, and only emits NotInRepository after repository detection definitively fails.

Concerns

• No blocking correctness, security, or spec-drift concerns found in the changed lines.
• The PR description includes a Loom recording, satisfying the repo visual-evidence requirement for this user-facing behavior.

Verdict

Found: 0 critical, 0 important, 0 suggestions

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[kevinchevalier]

Nice! This makes the state clear!