[rednafi]

Diff

• Use znkr.io/diff in place of go-udiff. Diffs match git, also for
  large rewrites. A file over 10,000 lines uses the fast diff.
• A file version over 2,000,000 bytes or 50,000 lines shows
  "file too large to show". A diff holds at most 128 MiB of text, and a
  change of more than 20,000 files shows only the count.

Speed

• Render lines only when they come into view, and highlight in the
  background with time limits. Slow lexers and very long lines show plain.
• Read blobs in parallel with a cache. List files, watch, and load the
  commit picker off the UI goroutine. Reloads wait less after an edit.
• Cap Markdown size, depth and work, PR text, find, and grep, so no
  input freezes the UI.

Layout

• A narrower file list, the full path above the diff, a thinner
  separator and gutter, and no rule above the footer.
• The left arrow closes an expanded fold. The hover tooltip is gone.

Fixes

• Scroll through a wrapped line taller than the pane. Keep the cursor
  visible in a one-row body and in a tall rendered block.
• Show invisible characters in names and code, and cut wide names to
  the list width.
• ] and [ step over removed blocks in rendered Markdown.
• The PR tab counts every check, and names the failed ones.
• Send a review over 64 KB in parts.
• -log files are private, hold no typed text, and a FIFO with no
  reader fails at once.
• -check validates -scope and -layout. A branch with no commits, a
  binary file, and tiny panes show clear notes.
• A file created and deleted around a reload leaves the list.

API

• diff: real Row, Fold, Kind and Side types; RenderFile takes
  FileOptions; SplitLines; always lazy.
• vcs: PR with Tally, File.Err, TooManyFilesError.Count, Watcher.Reset.
• rex: New and update.NewInstaller return (value, ok); Paste sends in
  parts; update.Restart takes env first.
• Code names follow the herdr-reviewr settings (navigator, hunk,
  scope, allFiles). Settings, keys, flags and resume state are unchanged.

Stacked on rel/1-internal. Merge in order.