make git.config_get behave more like dict.get

previously, calling git.config_get with an invalid key would throw a
CommandFailed exception. This commit modifies config_get so that it
will return a configurable default value (None by default) if it
cannot look up the requested configuration key.

Change-Id: I07b8b684b7d3cf3c4c50a00328e533e69c91822c

Author: larsks

rdopkg/utils/cmd.py

@@ -301,8 +301,16 @@ def get_file_content(self, rev, path):
         obj = '%s:%s' % (rev, path)
         return self('show', obj, log_cmd=False)
 
-    def config_get(self, param):
-        return self("config", "--get", param)
+    def config_get(self, param, default=None):
+        '''Return the value of a git configuration option.  This will
+        return the value of the default parameter (which defaults to
+        None) if the given option does not exist.'''
+
+        try:
+            return self("config", "--get", param,
+                        log_fail=False, log_cmd=False)
+        except exception.CommandFailed:
+            return default
 
     def config_set(self, param, value, is_global=False):
         params = [param, value]