Many diff commands use a non-zero return code to indicate that the given files

are different. This is fine, except we log the output of commands even when

we're expecting non-zero return codes. This was causing issues when the output

of the diff command wasn't ASCII, and we tried to format it into a logging

call.
We don't actually care about logging the output of diffs, since it makes our

debug output pretty verbose. This change adds a new argument and some plumbing

to make sure that we don't log the output of diff commands.

Ran unit tests.
Verified that diff contents were no longer logged with SCMs where they were

  previously (perforce, svn)