Fix discard unstaged changes incorrectly requiring staged changes

[dhanush0x96c]

Summary

Fixes #5388

This PR fixes the logic controlling when the "Discard unstaged changes" menu option is disabled in the files panel.

Previously, the option could be incorrectly disabled even when there were unstaged changes available to discard. This change ensures the option is only disabled when there are no unstaged changes.

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Problem

The previous condition disabled the menu item when either:

• The selected files had no staged changes, OR
• The selected files had no unstaged changes

This was overly restrictive. The availability of the action should depend solely on whether there are unstaged changes. As a result, users were prevented from discarding unstaged changes simply because no staged changes were present.

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Solution

The condition has been simplified:

• The menu item is now disabled only when there are no unstaged changes
• The presence or absence of staged changes is no longer considered

This aligns the UI behavior with user expectations and the intent of the action.

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Visual Changes

Before

• Unstaged changes present, but action incorrectly disabled

After

• Unstaged changes present, action correctly enabled

• No unstaged changes, action correctly disabled

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Testing

1. Stage some (but not all) changes in a file
2. Open the discard changes menu for that file
3. Confirm "Discard unstaged changes" is enabled
4. Stage all changes in a file
5. Confirm "Discard unstaged changes" is disabled with the updated message
6. Modify a file without staging any changes
7. Confirm "Discard unstaged changes" is enabled

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Checklist

• Cheatsheets are up-to-date (go generate ./...)
• Code has been formatted
• [] Tests have been added/updated
• Text is internationalised
• [] UserConfig changes (if any) support hot-reloading
• [] Documentation has been updated (if required)
• Reviewed changes for errors and inconsistencies

[dhanush0x96c]

Note: updated the English translation only; other languages are handled via Crowdin.

[themavik]

Disabled state now matches the action: unstaged discard only needs unstaged worktree deltas, not staged ones. Tooltip string lines up with the new check.