Prior to this change, it was not possible for the user to suppress warnings from git, mercurial, and perforce. This could result in a large number of warning messages output to the log which can be cumbersome and fill up the console window.
This change allows the user to suppress warnings from rbtools/clients/git.py, rbtools/clients/mercurial.py, and rbtools/clients/perforce.py by setting SUPPRESS_CLIENT_WARNINGS in .reviewboardrc to True.

Checked that the warning message in git.py is suppressed when the SUPPRESS_CLIENT_WARNINGS is set to True.