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-exec

@@ -18,7 +18,7 @@ set -e
 
 # Provide rbenv completions
 if [ "$1" = "--complete" ]; then
-  exec rbenv shims --short
+  exec rbenv-shims --short
 fi
 
 RBENV_VERSION="$(rbenv-version-name)"

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

libexec/rbenv-which

@@ -12,7 +12,7 @@ set -e
 
 # Provide rbenv completions
 if [ "$1" = "--complete" ]; then
-  exec rbenv shims --short
+  exec rbenv-shims --short
 fi
 
 remove_from_path() {