git_panel: Add copy path actions to the context menu (#62352)

# Objective

When developing remotely, when I close the uncommitted changes tab, I
need some time to load before I can copy the path. Or have to switch to
the project panel to find the specific file. All of this is annoying, so
I added a copy path action to the git panel's context menu and key
bindings consistent with the project panel.

## Solution

Already described in the Objective section.

## Testing

I wrote a unit test and tested it manually.

## Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content adheres to Zed's UI standards
([UX/UI](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
and
[icon](https://github.com/zed-industries/zed/blob/main/crates/icons/README.md)
guidelines)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

## Showcase

<img width="411" height="535" alt="showcase"
src="https://github.com/user-attachments/assets/a17e7633-eb92-4737-aa30-958fe58bb99f"
/>
<img width="717" height="427" alt="showcase"
src="https://github.com/user-attachments/assets/d067e892-17de-4527-ac20-16cad6f38015"
/>


---

Release Notes:

- Added "Copy Path" and "Copy Relative Path" actions to the Git Panel's
context menu