gnuplot output: Optimize by only storing changes.

Only output new lines in gnuplot output if there have been changes in
the samples (similar to what VCD does). As long as the first and last
sample are output, the resulting plot looks OK.

This reduces the size of the output file from roughly 200MB to just 60KB
in one specific test setup (depends on the number of probes and on the
signal, of course). The time and CPU load required to generate the gnuplot
output and the resulting plot (PNG or other) is also drastically reduced
from multiple minutes to roughly 30 seconds (again, depends on various
things).

Thanks Ken Mobley of ChronoVu for the report.