Fix preapply hooks are not called on no diff when run apply subcommand (#522)

* Fix presync hooks are not called on no diff when run apply subcommand

Signed-off-by: xiaomudk <xiaomudk@gmail.com>

* Update docs/index.md

Co-authored-by: Yusuke Kuoka <ykuoka@gmail.com>
Signed-off-by: xiaomudk <xiaomudk@gmail.com>

Signed-off-by: xiaomudk <xiaomudk@gmail.com>
Co-authored-by: Yusuke Kuoka <ykuoka@gmail.com>

pkg/app/testdata/testapply/noop/log

@@ -34,4 +34,11 @@ second-pass rendering result of "helmfile.yaml.part.0":
 merged environment: &{default map[] map[]}
 2 release(s) found in helmfile.yaml
 
+invoking preapply hooks for 2 groups of releases in this order:
+GROUP RELEASES
+1     default//foo
+2     default//bar
+
+invoking preapply hooks for releases in group 1/2: default//foo
+invoking preapply hooks for releases in group 2/2: default//bar
 changing working directory back to "/path/to"

pkg/app/testdata/testapply_hooks/hooks_for_no-diff_release/log

@@ -2,5 +2,8 @@
 hook[prepare] logs | foo
 hook[prepare] logs | 
 
+hook[preapply] logs | foo
+hook[preapply] logs | 
+
 hook[cleanup] logs | foo
 hook[cleanup] logs |