build-site: Fix push/commit parameter logic.
Commit and push site when appropriate.

Previous commit checkin didn't test that the site actually changed
before committing. Now we test that and commit/push only if
necessary.
images.rkt: Convert to use ImageMagick.

The algorithm used to scale the thumbnails was really bad for
some images, so let's use ImageMagick instead.
site.rkt: Fix chapter list hashes for index.html.

Apparently, the mustache parser wants strings without '-'.
index.ms: Fix template variable references

Mustache apparently doesn't like '-', so use '_' instead.
page.ms: Remove spaces around previous/next links.