imerge.css: Add some semicolons.
Only draw the final frontier.

Don't draw the frontier of each block, but only the final frontier
between known and unknown areas.
Implement table borders via cells rather than on the table as a whole.

This will make it easier to add more cells outside of the border.
imerge.css: Remove a redundant background setting.

These cells are already caught by "frontier_within".
Add a new class, "frontier_without", for cells outside of the frontier.

Color cells based on this class and "frontier_within".
Add table headers showing the indexes for each row and column.
Color the initial cells blue in the HTML output.
Fix the merge frontier where it touches the initial commits.
Add a constant Block.MERGE_MASK.

Use it to mask *in* the merge bits rather than using FRONTIER_MASK to
mask *out* the frontier bits.
Simplify test.
imerge.css: Rearrange rules.
HTML diagram: Distinguish skipped merges visually.
HTML diagram: Make unknown commits pale yellow.

This also makes it possible to tone down the red of BLOCKER commits.
Merge branch 'html-lipstick'

Improve the HTML diagram output:

* Desaturate the existing colors

* Add some new colors:

  * Blue for initial commits

  * Pale green for skipped commits (and with "-" instead of "?")

  * Pale yellow for unknown commits

* Display the numerical indexes along the top and left edges of the
  table.

* Fix how the frontier is drawn.