The "-m" doesn't do anything by itself, but it's there in case you want to add an option like "-p", which otherwise wouldn't show a useful diff for a merge comm

The "-m" doesn't do anything by itself, but it's there in case you want to add an option like "-p", which otherwise wouldn't show a useful diff for a merge commit. With "--first-parent -m -p", the log shows only the diff of the merge commit against its first parent, which is the same as what you would get from squashing.