clang_tidy_gerrit.py: add a local mode
clang_tidy_gerrit.py: add a local mode

This patch makes it possible to run clang_tidy_gerrit.py "locally" (i.e.
without posting anything to gerrit). The new argument parsing is such that
regular clang_tidy_gerrit.py runs should continue to work unmolested.

Here is some sample output:

  $ build-support/clang_tidy_gerrit.py -n HEAD
  42349 warnings generated.
  83338 warnings generated.
  Suppressed 83398 warnings (83331 in non-user code, 5 due to line filter, 61 NOLINT, 1 with check filters).
  Use -header-filter=.* to display errors from all non-system headers. Use -system-headers to display errors from system headers as well.
  INFO:root:Clang output
  INFO:root:/home/adar/Source/kudu/src/kudu/consensus/raft_consensus.cc:2253:69: warning: the parameter 'client_cb' is copied for each invocation but only used as a const reference; consider making it a const reference [performance-unnecessary-value-param]
                                                  StdStatusCallback client_cb,
                                                                    ^

  Skipping gerrit

Change-Id: I2910d32e4318eea4ca947c52717ee386063fa45f
Reviewed-on: http://gerrit.cloudera.org:8080/7528
Reviewed-by: Todd Lipcon <2c763d726a8c2f6afda9be7e0e56a2caa6bb5e6b@apache.org>
Tested-by: Adar Dembo <866ca730bfc7f544e3ebce604ceeba4879f23df0@cloudera.com>

andrwng/kudu,helifu/kudu,EvilMcJerkface/kudu,cloudera/kudu,andrwng/kudu,EvilMcJerkface/kudu,helifu/kudu,EvilMcJerkface/kudu,andrwng/kudu,cloudera/kudu,cloudera/kudu,EvilMcJerkface/kudu,cloudera/kudu,helifu/kudu,helifu/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,InspurUSA/kudu,andrwng/kudu,helifu/kudu,InspurUSA/kudu,andrwng/kudu,cloudera/kudu,EvilMcJerkface/kudu,cloudera/kudu,EvilMcJerkface/kudu,andrwng/kudu,cloudera/kudu,andrwng/kudu,cloudera/kudu,InspurUSA/kudu,InspurUSA/kudu,andrwng/kudu,andrwng/kudu,cloudera/kudu,helifu/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,helifu/kudu,InspurUSA/kudu,helifu/kudu,InspurUSA/kudu,InspurUSA/kudu,helifu/kudu,EvilMcJerkface/kudu