[rednafi]

Fixes #3. Stacked on #5.

Change

• A bar ▌ in the colour of the line (ANSI green or red) marks an inserted or a deleted line, in place of + or -. This is the style of herdr-reviewr.
• The bar takes the same two cells as a sign. The columns, the clicks, the wrap, and the scroll do not change.
• The setting diff_signs = true gives + and - back, for users who want marks that do not depend on colour.
• The review text that goes to the agent keeps + and -.
• The vendored diffview has the change. crush has no option for it, so VENDORED.md lists it.

   3  3   func A() int {          diff_signs = true:     3  3   func A() int {
   4    ▌     return 1                                   4    -     return 1
      4 ▌     x := 2                                        4 +     x := 2
      5 ▌     return x                                      5 +     return x

Tests

• diff.TestMarks: the exact screen lines with the bar and with signs, in the unified and the split layout; the colour of each mark (red for a delete, green for an insert); the bar with a scroll to the right (▌…cdefgh); the review text keeps + and - in both cases.
• tui.TestDiffSigns: the screen of rexdiff shows the bar by default, and +/- with {"diff_signs": true}.
• TestParseConfig accepts diff_signs.
• Checked in Rex with the built binary: unified, split, and diff_signs.

[rednafi]

Merged into master through #10.