Use command to run rbenv rehash

In the event that `eval "$(rbenv init -)"` is called from a function named
rbenv (which I do to get rbenv to load lazily in my shell), evaluating the
phrase `rbenv rehash` will cause the outer function to run again (causing an
infinite loop).

This change makes it clear you want the command named rbenv and not a function
which may exist in the environment.

libexec/rbenv-init

@@ -88,7 +88,7 @@ if [ -r "$completion" ]; then
 fi
 
 if [ -z "$no_rehash" ]; then
-  echo 'rbenv rehash 2>/dev/null'
+  echo 'command rbenv rehash 2>/dev/null'
 fi
 
 commands=(`rbenv-commands --sh`)

test/init.bats

@@ -14,7 +14,7 @@ load test_helper
 @test "auto rehash" {
   run rbenv-init -
   assert_success
-  assert_line "rbenv rehash 2>/dev/null"
+  assert_line "command rbenv rehash 2>/dev/null"
 }
 
 @test "setup shell completions" {