Selected hunk or line do not stay selected if a new changed line is added or removed above it

[smashwilson]

Prerequisites

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Description

In a MultiFilePatchView, the selection is not maintained correctly if any of the following situations:

• The diff is updated to account for external changes and hunks or lines are added above the current selection.
• A FilePatch is collapsed or expanded (collapse large diffs && collapse/expand any diff within multi file patch view #1913) above the current selection.

Steps to Reproduce

1. Open Atom on a git repository containing at least one modified file.
2. Open a ChangedFileItem on one file by clicking it on the git tab.
3. Select a changed line other than the first changed line.
4. Open a TextEditor on the same file.
5. Add a line above the selected change and save the file.

Expected behavior:

The same logical line within the diff should be selected within the ChangedFileItem.

Actual behavior:

The changed line above the previously selected line is selected.

Reproduces how often:

100%

Versions

github@0.26.0 (atom/github#2d67c14d)

Additional Information

There's a pretty hard sub-problem hiding within this one, which is that it's really hard to identify "the same line" between two Patch instances. We might need to do some kind of diff between the diffs?