Have the ka-lint on $PATH run webapp's ka-lint when appropriate!

Summary:
This is the final step of hooking up webapp's testing/ka-lint binary
to replace khan-linter for linting files within webapp.

The driver script that tools and people run manually, `ka-lint`, is
now a shell script that dispatches to either khan-linter's runlint.py
or webapp's testing/ka-lint, depending on where it's called from.

Issue: https://khanacademy.atlassian.net/browse/INFRA-4781

Test Plan:
I ran
   touch /tmp/README
   ka-lint /tmp/README
from both /tmp and from webapp-root.  I got different output -- the
second complained that you can only lint webapp files from within
webapp -- showing that it was dispatching correctly.

Reviewers: benkraft, davidbraley

Reviewed By: benkraft, davidbraley

Subscribers: kevinb

Differential Revision: https://phabricator.khanacademy.org/D63551

bin/ka-lint

@@ -0,0 +1,28 @@
+#!/bin/bash
+
+# A script that calls the proper runlint.py for the repo you are
+# currently in.
+#
+# For repos, such as webapp, that define their own ka-lint --
+# either at the top level or in <top-level>/testing -- we run
+# their ka-lint.  Otherwise we run runlint.py in khan-linter.
+#
+# Note that if you are in webapp-root, say, and run
+#    ka-lint /tmp/whatever
+# we will still use webapp's linter even though you are linting a file
+# outside webapp.  There's no great way to do otherwise -- what do we do
+# for
+#   ka-lint webapp_file.py /tmp/whatever
+# ?  Separate into separate ka-lint calls?  It's a mess.  And I
+# don't expect people to do this outside-repo linting anyway, so I'm not
+# going to worry about it for now.
+
+repo_root=$(git rev-parse --show-toplevel 2>/dev/null)
+
+if [ -n "$repo_root" -a -e "$repo_root/testing/ka-lint" ]; then
+    exec "$repo_root/testing/ka-lint" "$@"
+elif [ -n "$repo_root" -a -e "$repo_root/ka-lint" ]; then
+    exec "$repo_root/ka-lint" "$@"
+else
+    exec "$(dirname "${BASH_SOURCE[0]}")/../runlint.py" "$@"
+fi