[MNG-8165] Align mvn.sh script with mvn.cmd (#1648)

As one does check for .mvn directory in FS root while other does not, stops one level before.

---

https://issues.apache.org/jira/browse/MNG-8165

apache-maven/src/assembly/shared/init

@@ -34,11 +34,15 @@ find_maven_basedir() {
 (
   basedir=`find_file_argument_basedir "$@"`
   wdir="$basedir"
-  while [ "$wdir" != '/' ] ; do
+  while :
+  do
     if [ -d "$wdir"/.mvn ] ; then
       basedir=$wdir
       break
     fi
+    if [ "$wdir" == '/' ] ; then
+      break
+    fi
     wdir=`cd "$wdir/.."; pwd`
   done
   echo "$basedir"