Render revision graph in the background

[mstv]

Fixes #11700

Proposed changes

Have clear chains of revision grid updates (one for graph loading affecting RowCount, one for visible row range).
Get rid of several workarounds and of the mix-up of revision loading and graph-only operations.
Avoid unnecessary blocking the UI thread.

• Do not render the graph in the UI thread but in a background task with double buffering
  in order to avoid calling the potentially expensive RevisionGraphRow.BuildSegmentLanes from the UI thread
• Remove RevisionDataGridView.UpdateGraphAsync, just call RevisionGraph.CacheTo once with cancellation
  because CacheTo is needed for graph rendering only
• Remove UpdateVisibleRowRange call on unhandled OnCellPainting
  because there no need to for this additional trigger of graph update
• RevisionDataGridView: Remove special OnScroll handler which called Update from time to time
  because the revision graph rendering does not block the UI any longer

Screenshots

N/A

Test methodology

• manual
• adapted tests

Merge strategy

Please do not squash merge!

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✒️ I contribute this code under The Developer Certificate of Origin.

[gerhardol]

This seem to work OK, have not reviewed all details

[gerhardol]

+1 seem to work well, no obvious comments in the review
It is hard to review though, it is hard to understand why changes are done, what is better, how this is an improvement.

[mstv]

It is hard to review though, it is hard to understand why changes are done, what is better, how this is an improvement.

Yes, this was missing in the description. Updated.

[RussKie]

[RussKie]

Suggested change

	private void RenderGraphToCache(int rowIndex, int rowHeight)
	private void RenderRowToCache(int rowIndex, int rowHeight)