If set, "singlePack" will create a single GC pack file for all
objects reachable from refs/*. If not set, the GC pack will contain
object reachable from refs/heads/* and refs/tags/*, and the GC_REST
pack will contain all other reachable objects.
Change-Id: I56bcb6a9da2c10a0909c2f940c025db6f3acebcb
Signed-off-by: Terry Parker <tparker@google.com>