Fix non-sticky range select after clicking in staging view

[stefanhaller]

• PR Description

When clicking in a single-file diff view to enter staging (or custom patch editing, when coming from the commit files panel), and then pressing shift-down or shift-up to select a range, it would move the selected line rather than creating a range. Only on the next press would it start to select a range from there.

This is very similar to the fix we made for pressing escape in #3828.

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[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
Report missing for 052974b1	✅ 100.00%

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (052974b)	Report Missing	Report Missing	Report Missing
Head commit (a58770e)	50665	43616	86.09%

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 (#3998)	21	21	100.00%

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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[jesseduffield]

LGTM