Specify main as the default branch. Fixes #269.

Author: jaraco

tox.ini

@@ -26,8 +26,8 @@ deps =
 	diff-cover
 commands =
 	pytest {posargs} --cov-report xml
-	diff-cover coverage.xml --html-report diffcov.html
-	diff-cover coverage.xml --fail-under=100
+	diff-cover coverage.xml --compare-branch=origin/main --html-report diffcov.html
+	diff-cover coverage.xml --compare-branch=origin/main --fail-under=100
 
 [testenv:perf]
 use_develop = False