schhist2web: use a cache by default and give the user control over it.

- scripts/schhist2web (THUMB_OPTS): make shared pixels in thumbnails a
  bit darker
- scripts/schhist2web (usage): described the arguments
- scripts/schhist2web: new option  -c dir  to set the cache directory
- scripts/schhist2web: new option  -n  to remake the cache
- scripts/schhist2web: path name in new file detection was incorrect
- scripts/schhist2web: use "$index" instead of open-coding "$out/index.html"
- scripts/gitsch2ppm: changed temp directory name from _schdiff_a to
  _gitsch2ppm

scripts/gitsch2ppm

@@ -81,7 +81,7 @@ fi
 [ "$commit" != HEAD -o -f "$dir/$schem" ] || usage
 [ -d "$dir/.git" ] || usage
 
-tmp="$dir/../_schdiff_a"
+tmp="$dir/../_gitsch2ppm"
 sch="$tmp/$sdir"
 
 rm -rf "$tmp"