Benchmark reference size should expose overall GC chart

This will help to pinpoint memory leak or footprint regression.

The gc log files are archived for each benchmark, partial gc activity is visible in overview report but the all benchmark gc activity is more interesting.

For now I have generated manually the gc chart with gcviewer using url like:
https://benchmarks.nuxeo.com/build/878/reports/gc-1.png
https://benchmarks.nuxeo.com/build/902/reports/gc-1.png
https://benchmarks.nuxeo.com/build/902/reports/gc-2.png

but only for build > 800 and < 907.
This should be automated and direct link provide to reports.