Speed up obtaining exec/which/whence completions

Delegate to `rbenv-shims` instead of `rbenv shims` and therefore skip
going through the main `rbenv` executable again that would set up a lot
of the environment that was already set.

libexec/rbenv-whence

@@ -8,7 +8,7 @@ set -e
 # Provide rbenv completions
 if [ "$1" = "--complete" ]; then
   echo --path
-  exec rbenv shims --short
+  exec rbenv-shims --short
 fi
 
 if [ "$1" = "--path" ]; then