[SPARK-2747] git diff --dirstat can miss sql changes and not run Hive tests

dev/run-tests use "git diff --dirstat master" to check whether sql is changed. However, --dirstat won't show sql if sql's change is negligible (e.g. 1k loc change in core, and only 1 loc change in hive).

We should use "git diff --name-only master" instead.

Author: Reynold Xin <[email protected]>

Closes apache#1656 from rxin/hiveTest and squashes the following commits:

f5eab9f [Reynold Xin] [SPARK-2747] git diff --dirstat can miss sql changes and not run Hive tests.

Author: rxin

dev/run-tests

@@ -37,7 +37,7 @@ JAVA_VERSION=$($java_cmd -version 2>&1 | sed 's/java version "\(.*\)\.\(.*\)\..*
 # Partial solution for SPARK-1455. Only run Hive tests if there are sql changes.
 if [ -n "$AMPLAB_JENKINS" ]; then
   git fetch origin master:master
-  diffs=`git diff --dirstat master | awk '{ print $2; }' | grep "^sql/"`
+  diffs=`git diff --name-only master | grep "^sql/"`
   if [ -n "$diffs" ]; then
     echo "Detected changes in SQL. Will run Hive test suite."
     export _RUN_SQL_TESTS=true # exported for PySpark tests