add emacs shell-script tagging

for files that don't start with a #! or end in .sh, the added tags
are nice for emacs users to automatically switch to the right mode.

Change-Id: If4b93e106191bc744ccad8420cef20e751cdf902
diff --git a/lib/swift b/lib/swift
index 783ec75..5931acc 100644
--- a/lib/swift
+++ b/lib/swift
@@ -429,3 +429,7 @@
 
 # Restore xtrace
 $XTRACE
+
+# Local variables:
+# mode: shell-script
+# End: