Fix crash with drag selecting and staging

[stefanhaller]

• PR Description

Fix a crash in the staging view when clicking below the end of the diff, dragging upwards into the diff to select a range, and pressing space to stage.

Fixes #4479.

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[stefanhaller]

@jesseduffield I labeled this as "blocks-release" because it's a crash bug, and I think we always want to fix those as quickly as we can.

I tried to add a regression test for this, but we don't have a way to click-and-drag in integration tests yet. I tried to add this, but couldn't immediately get it to work; I'm out of time now, but let me know if you want me to try harder.

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
Report missing for 61636d81	✅ 80.00%

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (61636d8)	Report Missing	Report Missing	Report Missing
Head commit (825e5c2)	54394	47167	86.71%

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#4480)	5	4	80.00%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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Footnotes

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