control: show screens (CLI views stage 2a) !553

Stage 2a of #316, Ref #318. Stacked on !552.

• mr show, issue show, repo show, build show at a terminal are screens built by mrShowScreen, issueShowScreen, repoShowScreen, buildShowScreen.
• Legend actions follow state and access: mr rebase when behind, mr merge only when the gates pass, issue assign/label for writers, issue close for writers and the author.
• gitutil.DiffNumstat for the Files section; section.note carries its totals.
• System events in an Events section; comments in Discussion with their bodies.
• checksMark pending glyph is ◐.
• Piped output pinned per command in testdata/plain/; byte-identical.
• Users wiki and changelog describe screens.