post_deploy.regenerate_all: assume previous_version is non-None

Save us the trouble of 1) checking previous_version and 2) storing the
tuple in a variable.

Most of the functions in the post_deploy_tasks queue don't bother
checking previous_version and assume it's a valid VersionInfo instance
anyway.

post_deploy.py

@@ -51,13 +51,11 @@ def generate(previous_version):
 
 
 def regenerate_all(previous_version):
-  if previous_version:
-    ver_tuple = (
-      previous_version.bloggart_major,
-      previous_version.bloggart_minor,
-      previous_version.bloggart_rev,
-    )
-  if ver_tuple < BLOGGART_VERSION:
+  if (
+    previous_version.bloggart_major,
+    previous_version.bloggart_minor,
+    previous_version.bloggart_rev,
+  ) < BLOGGART_VERSION:
     regen = PostRegenerator()
     deferred.defer(regen.regenerate)