rev-list: Introduce --quiet to avoid /dev/null redirects

rev-list: Introduce --quiet to avoid /dev/null redirects

Some uses of git-rev-list are to run it with --objects to see if
a range of objects between two or more commits is fully connected
or not.  In such a case the caller doesn't care about the actual
object names or hash hints so formatting this data only for it to
be dumped to /dev/null by a redirect is a waste of CPU time.  If
all the caller needs is the exit status then --quiet can be used
to bypass the commit and object formatting.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>