Add support for running Checkstyle on a given SHA-1.

Usage e.g.: ./checkstyle.py -s 3aca02ca91816a86febb5ed6e380ec2122adea47

Additionally this CL adds a pre-push script that can be used a git
hook to run Checkstyle on 'repo upload'

Bug: 25852971
Change-Id: Ia00d48df80b2b024de0899d9590868016c5b7bf0

tools/checkstyle/pre-push.py

@@ -0,0 +1,48 @@
+#!/usr/bin/python
+
+#
+# Copyright 2016, The Android Open Source Project
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+
+"""Pre-push hook to run Checkstyle on changes that are about to be uploaded.
+
+  Usage: add a symbolic link from /path/to/git/repo/.git/hooks/pre-push to
+  this file.
+"""
+
+import sys
+import checkstyle
+
+
+def main():
+  print '\nStarting Checkstyle!\n'
+  sys.stdout.flush()
+
+  ins = raw_input()
+  (errors, warnings) = checkstyle.RunCheckstyleOnACommit(ins.split(' ')[1])
+
+  if errors or warnings:
+    print 'Upload anyway (y/N)?:'
+    sys.stdin = open('/dev/tty')
+    sys.stdout.flush()
+    answer = raw_input()
+    if 'y' == answer.lower():
+      sys.exit(0)
+    else:
+      sys.exit(1)
+
+
+if __name__ == '__main__':
+  main()