Addressing the issue of PRs not targetting the default branch

coverage_comment/main.py

@@ -119,11 +119,17 @@ def process_pr(
         base_ref=base_ref, coverage_path=config.COVERAGE_PATH
     )
 
-    previous_coverage_data_file = storage.get_datafile_contents(
-        github=gh,
-        repository=config.GITHUB_REPOSITORY,
-        branch=config.COVERAGE_DATA_BRANCH,
-    )
+    # It only really makes sense to display a comparison with the previous
+    # coverage if the PR target is the branch in which the coverage data is
+    # stored, e.g. the default branch.
+    previous_coverage_data_file = None
+    if base_ref == repo_info.default_branch:
+        previous_coverage_data_file = storage.get_datafile_contents(
+            github=gh,
+            repository=config.GITHUB_REPOSITORY,
+            branch=config.COVERAGE_DATA_BRANCH,
+        )
+
     previous_coverage = None
     if previous_coverage_data_file:
         previous_coverage = files.parse_datafile(contents=previous_coverage_data_file)