Merge pull request #1307 from native-api/ps4

Use a better PS4 as recommeneded by Bash Hackers Wiki

libexec/rbenv

@@ -7,7 +7,8 @@ if [ "$1" = "--debug" ]; then
 fi
 
 if [ -n "$RBENV_DEBUG" ]; then
-  export PS4='+ [${BASH_SOURCE##*/}:${LINENO}] '
+  # https://wiki-dev.bash-hackers.org/scripting/debuggingtips#making_xtrace_more_useful
+  export PS4='+(${BASH_SOURCE}:${LINENO}): ${FUNCNAME[0]:+${FUNCNAME[0]}(): }'
   set -x
 fi