Add config to use a pager in the staging panel

[stefanhaller]

• PR Description

Very rough, preliminary prototype of enabling a pager in the staging panel. It adds a separate config option for this, because the staging pager likely needs special command-line options (e.g. --patch in case of diff-so-fancy, --color-only in case of delta).

This is not for real use yet, it doesn't work well enough; it's just to see what problems we would have to solve if we wanted this.

Some of the problems:

• When line wrapping is on (default), there's sometimes a discrepancy between what's selected and what it acts on (can be seen when jumping between hunks with arrow left/right, it doesn't select the first changed line of the hunk, but one before). I suspect this may be caused by the Decolorize function not working well enough. For testing it's probably a good idea to disable wrapping (gui.wrapLinesInStagingView: false) to work around this.
• In the patch building panel it stops using the pager as soon as you stage a line. This is because it wants to color the line starts of the lines included in the patch; we'd need to figure out some other way of visualizing this.
• For delta, the highlighted line is almost invisible on added or deleted lines, because highlighting changes only the background color, but delta paints over the entire background for added and deleted lines. diff-so-fancy doesn't do this, so it works a little better there.

Would fix #4201, #2117, and #1939 if we got this to work for real.

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
Report missing for 4e38a941	✅ 36.36%

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (4e38a94)	Report Missing	Report Missing	Report Missing
Head commit (6fbd99a)	53129	45994	86.57%

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Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#4332)	44	16	36.36%

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