Filesets are relative to the current working directory by default, so we use
rootglob: to change this behavior.
As far as I know, set: isn't a pattern itself, it's a "hey, everything that comes after this is a fileset, not a glob".
I'm OK waiting for D7570, it's the main reason I split this out from the other two in this stack, but if we want to accept it before D7570 makes it in (unsure if it's currently blocked), I'm also OK with that.
I think that the later commits are still useful, though: I don't think D7570 deals with listfile, and the configuration for clang-format is broken in other ways. I'll reorder them so this is tip instead of root.
Oops, split failure, this line should remain, I'll send an update.