fix: preserve line order when staging non-contiguous lines in change blocks

[elskow]

Fixes #5103

When you have a diff with consecutive deletions followed by additions (like renaming a variable across multiple lines), and you try to stage only some of them (e.g., first deletion + first addition), the resulting patch had lines in the wrong order.

The problem was that unselected deletions were being converted to context lines immediately, causing them to appear between the selected deletion and addition. Now we defer those context lines until after all additions in the block.

Before:

-  // init something
   int i = 0;
+  // init variables

After:

-  // init something
+  // init variables
   int i = 0;

[Copilot]

Pull request overview

This PR fixes a bug in patch transformation where staging non-contiguous lines within change blocks (multiple consecutive deletions followed by additions) would not preserve the correct line order. The fix introduces special handling for change blocks to ensure that selected deletions and additions are kept together as pairs, while unselected deletions are converted to context lines.

Key Changes:

• Added logic to detect and handle change blocks (multiple consecutive deletions) as a unit
• Selected deletions and additions are now processed together before appending unselected lines as context
• New test case validates that non-contiguous selection preserves change pair relationships

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
pkg/commands/patch/transform.go	Implemented change block detection and special handling for non-contiguous line selection in the transformHunkLines function
pkg/commands/patch/patch_test.go	Added test case TestTransformNonContiguousSelection to verify correct behavior when selecting non-contiguous lines in change blocks

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[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

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[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

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[elskow]

@stefanhaller mind taking a look?