Block files without trailing newlines

Add a pre-submit check to ensure that files have a newline character at
the end.

Please see https://android.googlesource.com/platform/tools/repohooks/
for documentation on how PREUPLOAD hooks work.

Test: created a change and watched the presubmit check reject it.
Change-Id: Id0528cb1bd6fa9c4483ba43720839832f4fec34d

PREUPLOAD.cfg

@@ -0,0 +1,2 @@
+[Hook Scripts]
+whitespace = tools/whitespace.sh ${PREUPLOAD_FILES}

tools/whitespace.sh

@@ -0,0 +1,13 @@
+#!/bin/bash
+
+RESULT=0
+
+for i in "$@"; do
+  a="`tail -c 1 "$i"`"
+  if [ "$a" != "" ]; then
+    echo "$i does not have a trailing newline" 1>&2
+    RESULT=-1
+  fi
+done
+
+exit $RESULT