upload: Split populate reviews into 2 phases

This sets the function up to allow arbitrary topic
order in the future.

The first pass handles finding the relative topic, but
does not assume it's already populated. The second pass
needs to assume that the relative is already populated
since it uses it, so it needs to be done in topological
order.

revup/topic_stack.py

@@ -513,13 +513,18 @@ async def populate_reviews(
 
             if relative_topic:
                 topic.relative_topic = self.topics[relative_topic]
+
+            seen_topics[name] = topic
+
+        for name, topic in list(self.topics.items()):
+            if topic.relative_topic:
                 if len(topic.tags[TAG_BRANCH]) == 0:
                     topic.tags[TAG_BRANCH].update(topic.relative_topic.tags[TAG_BRANCH])
                 elif not topic.tags[TAG_BRANCH].issubset(topic.relative_topic.tags[TAG_BRANCH]):
                     raise RevupUsageException(
                         f"Topic {name} has branches"
                         f" {topic.tags[TAG_BRANCH] - topic.relative_topic.tags[TAG_BRANCH]} not in"
-                        f" relative topic {relative_topic}"
+                        f" relative topic {topic.relative_topic.name}"
                     )
 
                 if len(topic.tags[TAG_RELATIVE_BRANCH]) == 0:
@@ -531,8 +536,8 @@ async def populate_reviews(
                     != topic.relative_topic.tags[TAG_RELATIVE_BRANCH]
                 ):
                     raise RevupUsageException(
-                        f"Topic {name} and relative topic {relative_topic} have differing relative "
-                        f"branches, {topic.tags[TAG_RELATIVE_BRANCH]} vs "
+                        f"Topic {name} and relative topic {topic.relative_topic.name} have "
+                        f"differing relative branches, {topic.tags[TAG_RELATIVE_BRANCH]} vs "
                         f"{topic.relative_topic.tags[TAG_RELATIVE_BRANCH]}"
                     )
             else:
@@ -571,7 +576,7 @@ async def populate_reviews(
             ):
                 raise RevupUsageException(
                     f"Topic {name} has uploader '{topic.tags[TAG_UPLOADER]}' while relative topic"
-                    f" {relative_topic} has uploader"
+                    f" {topic.relative_topic.name} has uploader"
                     f" {topic.relative_topic.tags[TAG_UPLOADER] or '{}'}"
                 )
             topic_uploader = min(topic.tags[TAG_UPLOADER]) if topic.tags[TAG_UPLOADER] else uploader
@@ -633,8 +638,6 @@ async def populate_reviews(
             if auto_add_users in ("a2r", "both"):
                 topic.tags[TAG_REVIEWER].update(topic.tags[TAG_ASSIGNEE])
 
-            seen_topics[name] = topic
-
         if limit_topics:
             for name in limit_topics:
                 if name not in self.topics: