Handle null in ProgressMonitor setters

Handle null in ProgressMonitor setters

These commands' monitor fields can never be null unless someone passes
null to setProgressMonitor.  Anyone passing null probably meant to
disable the ProgressMonitor, so do that (by falling back to
NullProgressMonitor.INSTANCE) instead of saving a null and eventually
producing NullPointerException.

Change-Id: I63ad93ea8ad669fd333a5fd40880e7583ba24827
Signed-off-by: Jonathan Nieder <jrn@google.com>