Fix coloring of tab markers in syntax highlighting

If syntax highlighting is enabled the visual tab markers might adopt
the surrounding code's coloring due to prettify pushing its own CSS
class inside of our vt span.  Override all of the prettify classes
when inside of a vt span with our own concept of the correct color.

Change-Id: I15ed9eb7348d5c4fb26f98332caed0194c6daefe
Signed-off-by: Shawn O. Pearce <sop@google.com>