update vcs option cascade

- use cfg.commit explicitly (this condition was previously only
  implicit, because the function isn't called in the first place
  if cfg.commit is False
- allow push even with tag=False

src/bumpver/vcs.py

@@ -225,6 +225,7 @@ def commit(
     new_version   : str,
     commit_message: str,
 ) -> None:
+    if cfg.commit:
         for filepath in filepaths:
             vcs_api.add(filepath)
 
@@ -233,7 +234,7 @@ def commit(
     if cfg.commit and cfg.tag:
         vcs_api.tag(new_version)
 
-    if cfg.commit and cfg.tag and cfg.push:
+    if cfg.commit and cfg.push:
         vcs_api.push(new_version)