Enable _ka-commit-lint option from bin

Summary:
To enable calling ka-lint from commit-msg githook, instead of directly
linking to the githook.py entry point.

For an example of how I'm thinking of using this, see the sample
commit-msg hook from the WIP ka-clone project:
https://github.com/Khan/ka-clone/blob/wip/templates/commit-lint

Test Plan:
- manually pass sample commit msg files to bin/_ka-commit-lint
- manually pass sample commit msg files to githook.py

Reviewers: marcos, alpert, csilvers

Reviewed By: csilvers

Subscribers: ethan, benkomalo

Differential Revision: https://phabricator.khanacademy.org/D18179

README

@@ -4,7 +4,7 @@ It uses existing linting tools (pep8, pyflakes, jshint, etc), but has been
 modified to suit Khan Academy's slightly different style guides.
 
 This repository also contains a wrapper scripts that can be used as a
-pre-commit hook for Git or Mercurial.
+pre-commit hook for Git.
 
 Installation
 ============
@@ -31,20 +31,11 @@ for more options.
 
 Automatic
 ---------
-You can automatically invoke lints on modified or added files in
-Mercurial.  Add to .hgrc to use:
-
-    [hooks]
-    pretxncommit.lint = /path/to/khan-linter/hghook.py
-
-For git hooks, dotfiles probably already took care of it for you.
-
 You can update the blacklist file in this repository to control what
 files should not be linted at all.  Alternately, you can create a
 blacklist of your own, and use the --blacklist_file flag to runlint.py
 (you'll have to modify hghook.py or githook.py to pass in the name of the
 blacklist file as well).
 
 To suppress the lint check, set the environment variable FORCE_COMMIT
-to 1 prior to calling `hg commit` (there doesn't seem to be a way to
-pass args to a pretxncommit script otherwise).
+to 1 prior to calling `git commit`.

bin/_ka-commit-lint

@@ -0,0 +1 @@
+../githook.py

githook.py

@@ -26,12 +26,12 @@ def _normalized_commit_message(text):
     return ''.join(lines).strip()
 
 
-def main():
+def main(commit_message_file):
     """Run a git pre-commit lint-check."""
     # If we're a merge, don't try to do a lint-check.
     git_root = subprocess.check_output(['git', 'rev-parse', '--git-dir'])
-
-    commit_message = open(sys.argv[1]).read()
+    # read the commit message contents from the file specified
+    commit_message = open(commit_message_file).read()
     # Get rid of the comment lines, and leading and trailing whitespace.
     commit_message = _normalized_commit_message(commit_message)
 
@@ -112,4 +112,4 @@ def main():
 if __name__ == '__main__':
     suppress_lint = os.getenv('FORCE_COMMIT', '')
     if suppress_lint.lower() not in ('1', 'true'):
-        sys.exit(main())
+        sys.exit(main(sys.argv[1]))