Command listings start their non-output lines with (gdb) or >
for multi-line commands. gdbinit files on the other hand don't
use either, but also don't show any output.

The existing GDB highlighting file is renamed to gdb-bt as it only
highlights backtraces. It is reused by the new GDB highlighter
to pretty-print outputs of bt-producing commands such as bt, where,
up, down etc.