perf: Fixup ordering, fix pathing, docs (#38970)

Release Notes:

- N/A

tooling/perf/src/lib.rs

@@ -1,5 +1,7 @@
 //! Some constants and datatypes used in the Zed perf profiler. Should only be
 //! consumed by the crate providing the matching macros.
+//!
+//! For usage documentation, see the docs on this crate's binary.
 
 use collections::HashMap;
 use serde::{Deserialize, Serialize};
@@ -274,7 +276,7 @@ impl Output {
                         continue;
                     };
                     let shift =
-                        (s_timings.iters_per_sec(s_iters) / o_timings.iters_per_sec(o_iters)) - 1.;
+                        (o_timings.iters_per_sec(o_iters) / s_timings.iters_per_sec(s_iters)) - 1.;
                     if shift > max {
                         max = shift;
                     }

tooling/perf/src/main.rs

@@ -218,7 +218,10 @@ fn compare_profiles(args: &[String]) {
     let ident_new = args.first().expect("FATAL: missing identifier for new run");
     let ident_old = args.get(1).expect("FATAL: missing identifier for old run");
     let wspace_dir = std::env::var("CARGO_MANIFEST_DIR").unwrap();
-    let runs_dir = PathBuf::from(&wspace_dir).join(consts::RUNS_DIR);
+    let runs_dir = PathBuf::from(&wspace_dir)
+        .join("..")
+        .join("..")
+        .join(consts::RUNS_DIR);
 
     // Use the blank outputs initially, so we can merge into these with prefixes.
     let mut outputs_new = Output::blank();