gc.sh: replace 'disabling bitmap writing' warning kludge

gc.sh: replace 'disabling bitmap writing' warning kludge

When show_progress was not set, we had a kludge to suppress the

  warning: disabling bitmap writing, as some objects are not being packed

message by grepping it out of the output as it was likely to occur
whenever a fork was repacked.  Instead just disable bitmap creation
whenever a non-empty alternates file is present since even if all
objects in the fork end up being packed and a bitmap is generated, the
client will end up seeing a warning about extra bitmaps being ignored
since Git can only use one bitmap and the top-level parent of the fork
will almost certainly have one.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>