Fix scroll position when entering staging view by clicking in the main view

[stefanhaller]

This is both a bug fix and a behavior change:

• The bug fix is that when entering the staging view for a selected file by clicking in the main view, and the main view was scrolled down, then it would scroll to the top and show an ugly range selection from the line that was clicked to the line that is now under the mouse after scrolling up.
• The behavior change is that when leaving the staging view by pressing escape, it now retains its scroll position rather than scrolling back up to the top.

In addition, this fixes a minor flickering issue when leaving the staging view.

Note that maintaining the scroll position when going into and out of the staging view is not always perfect for two reasons:

• the focused staging view does not wrap long lines, but the unfocused one does
• a pager like delta might change the number of lines e.g. of the diff hunks

For the second problem there's little we can do, but the first one will be improved once we wrap lines in the focused staging view (see #1384 and #3558), which I'm currently working on.

• Please check if the PR fulfills these requirements

• Cheatsheets are up-to-date (run go generate ./...)
• Code has been formatted (see here)
• Tests have been added/updated (see here for the integration test guide)
• Text is internationalised (see here)
• If a new UserConfig entry was added, make sure it can be hot-reloaded (see here)
• Docs have been updated if necessary
• You've read through your own file changes for silly mistakes etc

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
Report missing for 4883c861	✅ 88.89%

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (4883c86)	Report Missing	Report Missing	Report Missing
Head commit (f08b3e9)	50663	43613	86.08%

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 (#3992)	18	16	88.89%

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%

See your quality gate settings    Change summary preferences

_{Codacy stopped sending the deprecated coverage status on June 5th, 2024. Learn more}

Footnotes

1. Codacy didn't receive coverage data for the commit, or there was an error processing the received data. Check your integration for errors and validate that your coverage setup is correct. ↩

[jesseduffield]

LGTM