Add support for the --stdin/etc flags, to match webapp's ka-lint.

Summary:
As part of the move of ka-lint from khan-linter to webapp, I'd like to
make the two drop-in replace-able with each other.  That means they
need to support the same flags -- or, at least, that this old version
supports all the flags that the new version does.

This commit adds that.  I add `--stdin` for real, and I add `--fix`
and `--fast` as noops.

Test Plan:
In webapp, I ran
   ka-lint -v main.py
   ka-lint -v --fast --fix main.py
   echo main.py | ka-lint -v --stdin
and they all succeeded with success, having linted one file.

I then ran
   ka-lint -v
   ka-lint -v --stdin </dev/null
and they both exited without doing any work.

Reviewers: benkraft, davidbraley

Reviewed By: benkraft, davidbraley

Subscribers: kevinb

Differential Revision: https://phabricator.khanacademy.org/D63417

runlint.py

@@ -817,7 +817,8 @@ def main(files_and_directories,
                                 % (num_new_errors, elapsed))
         except Exception:
             _get_logger().error(u"ERROR linting %r with %s:\n%s" % (
-                          files, type(lint_processor), traceback.format_exc()))
+                          files, type(lint_processor),
+                          traceback.format_exc().decode('utf-8')))
             num_framework_errors += 1
             continue
 
@@ -877,11 +878,29 @@ def main(files_and_directories,
                       default=False,
                       help=('Propose patches that arc can apply to fix lint'
                             'errors. Only useful when used with phabricator.'))
+    parser.add_option('--stdin', action='store_true', default=False,
+                      help=('Read list of files from stdin rather than '
+                            'the commandline.  We do not auto-pull in '
+                            'this case.'))
+    parser.add_option('--fast', action='store_true', default=False,
+                      help=("Ignored; for compatibility with webapp's "
+                            "ka-lint"))
+    parser.add_option('--fix', action='store_true', default=False,
+                      help=("Ignored; for compatibility with webapp's "
+                            "ka-lint"))
     parser.add_option('--verbose', '-v', action='store_true', default=False,
                       help='Print information about what is happening.')
 
     options, args = parser.parse_args()
-    if not args:
+
+    if options.stdin:
+        filenames = sys.stdin.read().splitlines()
+        # We can't auto-pull because we'd lose stdin when we re-exec'ed.
+        options.no_auto_pull = True
+    else:
+        filenames = args
+
+    if not filenames:
         # We used to lint the whole directory-tree when args was null,
         # but we want to be able to run
         #   git ls-files ... | xargs runlint.py
@@ -903,7 +922,7 @@ def main(files_and_directories,
 
     # normal operation
     (num_lint_errors, num_framework_errors) = main(
-        args,
+        filenames,
         options.blacklist, options.blacklist_filename,
         options.extra_linter, options.lang, options.verbose,
         options.propose_arc_fixes)