git_ui: Add stash toggle to git graph

[TunaDev0]

Objective

• Make stash entries in the Git Graph configurable instead of always showing them.
• This PR does not fix a tracked issue.

Solution

• Add a new persistent setting: git_panel.show_stashes_in_graph.
• Add a Settings UI entry so the behavior can be configured from the Git Panel settings.
• Add a Show stashes toggle directly in the Git Graph UI and in the graph header menu for quicker access.
• Filter refs/stash entries out of the Git Graph when the setting is disabled.
• Refresh the graph when the setting changes, and only react to stash-change graph invalidation when stash entries are currently visible.

Testing

• Tested with:
  • cargo check -p git_ui -p settings_ui -p settings_content
  • cargo test -p git_ui test_git_graph_hides_stashes_when_setting_disabled -- --nocapture
  • cargo test -p git_ui test_graph_data_reloaded_after_stash_change -- --nocapture
• Added automated coverage for:
  • hiding stash commits when the setting is disabled
  • reloading graph data after stash changes
• Reviewers can test by:
  • opening the Git Graph for a repository with stashes
  • toggling Show stashes from the graph UI
  • confirming stash entries appear/disappear immediately
  • changing the same option in Settings and confirming the graph updates
• Tested on:
  • macOS
• Not tested on:
  • Linux
  • Windows

Self-Review Checklist:

• I've reviewed my own diff for quality, security, and reliability
• Unsafe blocks (if any) have justifying comments
• The content adheres to Zed's UI standards (UX/UI and icon guidelines)
• Tests cover the new/changed behavior
• Performance impact has been considered and is acceptable

Showcase

• Adds a Show stashes toggle to the Git Graph so stash commits can be shown or hidden without leaving the view.