Comment for compute_rerun

Summary: This took a little figuring out when I was reading the code, so figured I'd add a comment.

Reviewed By: ndmitchell

Differential Revision: D53361033

fbshipit-source-id: 90c12ecd4400920d49548f87d53adde104ba0666

btd/src/lib.rs

@@ -266,6 +266,9 @@ pub fn main(args: Args) -> anyhow::Result<()> {
     Ok(())
 }
 
+/// Tells us which things might have changed, and therefore what
+/// we should run buck2 targets on at the diff revision to
+/// properly check if it really did change.
 fn compute_rerun(
     base: &Targets,
     changes: &Changes,