RFR (S): 8228388: Add information about dirty/skipped card for Merge HCC in G1 log

[Thomas Schatzl]

Hi all,

  can I have reviews for this small change that adds information about
how many cards were flushed/used from the HCC during GC?

This removes one annoyance that otherwise the amount of scanned cards
in the scan heap roots phase looks to be larger than the sum of cards
from the remembered sets and log buffers at times.

Also I need that value for JDK-8227739 (so far).

I added a small test case that checks whether G1 survives a GC without
HCC enabled - there is no such test yet, but seemed appropriate.

CR:
https://bugs.openjdk.java.net/browse/JDK-8228388
Webrev:
http://cr.openjdk.java.net/~tschatzl/8228388/webrev/
Testing:
hs-tier1, local gc/g1 run, new test, eyeballing log message output

Thanks,
  Thomas