[SPARK-2233] make-distribution script should list the git hash in the RELEASE file

This patch adds the git revision hash (short version) to the RELEASE file. It uses git instead of simply checking for the existence of .git, so as to make sure that this is a functional repository.

Author: Guillaume Ballet <gballet@gmail.com>

Closes #1216 from gballet/master and squashes the following commits:

eabc50f [Guillaume Ballet] Refactored the script to take comments into account.
d93e5e8 [Guillaume Ballet] [SPARK 2233] make-distribution script now lists the git hash tag in the RELEASE file.

make-distribution.sh

@@ -100,6 +100,14 @@ if [ -z "$JAVA_HOME" ]; then
   exit -1
 fi
 
+if which git &>/dev/null; then
+    GITREV=$(git rev-parse --short HEAD 2>/dev/null || :)
+    if [ ! -z $GITREV ]; then
+	 GITREVSTRING=" (git revision $GITREV)"
+    fi
+    unset GITREV
+fi
+
 if ! which mvn &>/dev/null; then
     echo -e "You need Maven installed to build Spark."
     echo -e "Download Maven from https://maven.apache.org/"
@@ -186,7 +194,7 @@ ${BUILD_COMMAND}
 # Make directories
 rm -rf "$DISTDIR"
 mkdir -p "$DISTDIR/lib"
-echo "Spark $VERSION built for Hadoop $SPARK_HADOOP_VERSION" > "$DISTDIR/RELEASE"
+echo "Spark $VERSION$GITREVSTRING built for Hadoop $SPARK_HADOOP_VERSION" > "$DISTDIR/RELEASE"
 
 # Copy jars
 cp $FWDIR/assembly/target/scala*/*assembly*hadoop*.jar "$DISTDIR/lib/"