fix(VisibleLinesCount): Consider HScrollBar

[mstv]

Fixes search hit being hidden by HScrollBar, also seen with git-grep

Proposed changes

• TextView.FirstVisibleLine: Replace setter with SetFirstVisibleLine
  which considers possibleHeightLoss because the horizontal scrollbar might become visible
• TextAreaControl.ScrollTo: Calculate completelyVisibleLinesCount and consider that the horizontal scrollbar might become visible

Screenshots

STR:

• select commit ae1340d3614c5affab0d828946c44e5e694d4bcd
• select file src/app/GitUI/UserControls/FileStatusDiffCalculator.cs
• search for "without HEAD"

Before

After