Auto return to the tree view if the last hunk in a file was staged

[WaterWhisperer]

This Pull Request fixes/closes #2748.

It changes the following:

• auto return to the tree view after staging the last hunk from the status diff view

2026.04.12.16.40.45.webm

I followed the checklist:

• I added unittests
• I ran make check without errors
• I tested the overall application
• I added an appropriate item to the changelog

[WaterWhisperer]

About unittests: I started by adding simple unit tests to the new helper function, but eventually removed it because it was redundant and affected the code layout of the status.rs itself.

[WaterWhisperer]

#2748 also mentions the issue of jumping to the directory level instead of the first file in the directory, which is the file that follows, but I think it depends on personal habits, and in some cases it is necessary to stage the entire folder directly, and I feel that this is reasonable behavior for the user, "if I want to stage the file separately then I just need to move it to it", so I didn't make any changes to this part