Unfortunately it is not possible to comment on a line in Github without a commit.
But you can create links to lines in your file (click on line and copy link on browser bar). For example you can use this links in your issues to discuss about code line:
https://github.com/sebastianbergmann/phpunit/blob/master/src/Runner/StandardTestSuiteLoader.php#L72
Update
You can also hold shift to obtain a link to a block of lines