fix(context_chips): stop GitDiffStats flicker from shell fallback

[zerone0x]

Summary

Fixes #9228.

When a per-repo GitRepoStatusModel is attached to CurrentPrompt, the GitDiffStats chip is updated via filesystem-watcher events whose DiffStateModel::diff_metadata_against_head() count includes untracked files (it walks git status --untracked-files=all).

However, every time the prompt context is rebuilt (e.g. when a new block's metadata arrives after a command completes), run_chips was still firing the chip's periodic shell-based generator once. For GitDiffStats that fallback is shell_git_line_changes(), which runs git diff --shortstat HEAD and counts only tracked changes. Its result was overwriting the watcher's structured GitLineChanges value until the next watcher MetadataChanged event restored it — causing the chip to visibly flicker between the tracked-only count and the all-files count whenever untracked files were present.

Fix

In CurrentPrompt::run_chips, when a chip is is_updated_externally, only run an initial_value_generator if one is provided. The shell-based chip.generator() is no longer used as a fallback for externally-driven chips:

• ShellGitBranch continues to seed its initial value from current_environment.git_branch() via initial_value_generator, then receives updates from the watcher.
• GitDiffStats has no initial_value_generator, so the periodic shell command is now skipped entirely. Initial population is handled by the watcher: GitRepoStatusModel::new() kicks off refresh_metadata immediately and emits MetadataChanged once metadata is computed, which set_git_repo_status handles by updating GitDiffStats with the structured GitLineChanges. Existing chip values survive across clear_chips() (which only aborts in-flight generators), so there is no transient empty state on subsequent prompt refreshes.

This keeps the diff-count semantics consistent (always all-files) without changing the behavior of remote/non-watched sessions, which still fall back to the shell generator via the periodic timer branch.

Test plan

• In a repo with both modified tracked files and untracked files, run a command (e.g. ls) and observe the GitDiffStats chip — it should stay on the all-files count without flickering down to the tracked-only count.
• In a repo without the watcher available (e.g. remote session), confirm the chip still updates periodically via the shell fallback (unchanged code path).
• ShellGitBranch continues to update on branch changes via the watcher and shows the correct branch on first paint.

CHANGELOG-BUG-FIX: Fix git diff chip flickering between tracked-only and all-files count when untracked files are present

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@zerone0x

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@vkodithala

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[vkodithala]

Was looking into this earlier - this is the right fix! Verified that this all works locally. Make sure that you sign our CLA and you're good to go!

[captainsafia]

@zerone0x Thanks for opening this PR! When you get a chance, please sign the CLA so we can move forward with merging this PR.

[zerone0x]

signed!

[oz-for-oss]

Overview

This PR prevents externally-driven periodic context chips from falling back to their shell generator when a watcher is already responsible for updates. That keeps GitDiffStats on the watcher-provided all-files semantics and preserves ShellGitBranch's prompt-context initial value path.

Concerns

• No blocking concerns found in the changed hunk.

Verdict

Found: 0 critical, 0 important, 0 suggestions

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