gitblit: use incremental GC and add -Xms

- use the incremental GC instead of G1,
  as suggested by Giuseppe

- set -Xms to the same value as -Xmx

Change-Id: I433ecc1e929adcbe36c566705249ec5de92bbece
Merge "gitblit: use incremental GC and add -Xms" into production