Export PS4 when RBENV_DEBUG is set for more informative debug output

libexec/rbenv

@@ -1,7 +1,11 @@
 #!/usr/bin/env bash
 set -e
-[ -n "$RBENV_DEBUG" ] && set -x
 
+
+if [ -n "$RBENV_DEBUG" ]; then
+  export PS4='+ [${BASH_SOURCE##*/}:${LINENO}] '
+  set -x
+fi
 resolve_link() {
   $(type -p greadlink readlink | head -1) "$1"
 }