Running coverage test on https://gerrit.wikimedia.org/r/c/mediawiki/extensions/CirrusSearch/+/510815 shows this in the logs:
13:19:08 PHP Notice: Undefined index: R in /opt/phpunit-patch-coverage/vendor/mediawiki/phpunit-patch-coverage/src/Git.php on line 57
13:19:08 PHP Fatal error: Uncaught Error: Cannot access empty property in /opt/phpunit-patch-coverage/vendor/mediawiki/phpunit-patch-coverage/src/Git.php:59
13:19:08 Stack trace:
13:19:08 #0 /opt/phpunit-patch-coverage/vendor/mediawiki/phpunit-patch-coverage/src/CheckCommand.php(161): MediaWiki\Tool\PatchCoverage\Git->getChangedFiles('7e2c08d35ff4667...')
13:19:08 #1 /opt/phpunit-patch-coverage/vendor/symfony/console/Command/Command.php(255): MediaWiki\Tool\PatchCoverage\CheckCommand->execute(Object(Symfony\Component\Console\Input\ArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))
13:19:08 #2 /opt/phpunit-patch-coverage/vendor/symfony/console/Application.php(960): Symfony\Component\Console\Command\Command->run(Object(Symfony\Component\Console\Input\ArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))
13:19:08 #3 /opt/phpunit-patch-coverage/vendor/symfony/console/Application.php(255): Symfony\Component\Console\Application->doRunCommand(Object(MediaWiki\Tool\PatchCoverage\CheckCommand), Object(Symfony\Component\Console\Input\ArgvInput), Object(Symfo in /opt/phpunit-patch-coverage/vendor/mediawiki/phpunit-patch-coverage/src/Git.php on line 59