test-grep: document that trivially moved lines might not be detected
I'm not going to fix this. I just learned "grep --diff" does clever thing
than a simple grep(patch.diff(context=0)), and I think it's better to
document how things work.
fn main() {
cc::Build::new()
.warnings(true)
.file("src/sendfds.c")
.file("src/sighandlers.c")
.compile("procutil");
}