Make pyenv init --path usable as a pyenv init - substitute

We're going to eliminate the need to mandatorily use `pyenv init --path`.
We can't delete it yet for backward compatibility.
Besides, there's one other use case for it: to enable shims but without
shell integration, e.g. for noninteractive shells.

To be a full-fledged replacement for `pyenv init -` however,
it needs to do rehashing.

test/init.bats

@@ -17,6 +17,13 @@ load test_helper
   assert_line "command pyenv rehash 2>/dev/null"
 }
 
+@test "auto rehash for --path" {
+  run pyenv-init --path
+  assert_success
+  assert_line "command pyenv rehash 2>/dev/null"
+}
+
+
 @test "setup shell completions" {
   root="$(cd $BATS_TEST_DIRNAME/.. && pwd)"
   run pyenv-init - bash