By JGit's default, git-receive-pack will run auto gc after receiving
data from git-push and updating refs.
Disable this behavior in Gerrit init to avoid the additional load it
creates: gc should be configured in gc config section or run as a
separate process.
Change-Id: I9450f8edbade7b6e789387645cfe70daa0949aff
53528f49a5