Add jj subcommands to easily compare revisions.

* `jj before-after` builds two copies of gn in parallel, before and after a change.
* `HYPERFINE_OPTS="--warmup 1 --runs 5" jj compare-perf` compares the performance of those two copies of GN across 5 invocations of GN gen each.
  * It caches the benchmarking results to improve performance, which
    particularly helps given that "before" is unlikely to change.

Change-Id: Iff7ef7965ebf7c1b9d2769ca23abdac36a6a6964
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/25720
Commit-Queue: Matt Stark <msta@google.com>
Reviewed-by: Takuto Ikuta <tikuta@google.com>

tools/jj/config.toml

diff --git a/tools/jj/config.toml b/tools/jj/config.toml
index 57413ed..9546641 100644
--- a/tools/jj/config.toml
+++ b/tools/jj/config.toml
@@ -6,6 +6,7 @@
 
 [revset-aliases]
 "trunk()" = "main@origin"
+"before" = "trunk()"
 
 # For users of jj-on-piper, this is enabled by default for *all* repos.
 # We want GN's formatters, not google3's.
@@ -16,6 +17,41 @@
 command = ["/dev/null"]
 patterns = ["null"]
 
+[aliases]
+compare = [
+  "run",
+  "-j2",
+  "--revision=before=before",
+  "--revision=after=@",
+  # Allow running on immutable commits
+  "--ignore-changes",
+  "--",
+  "bash",
+  "-c",
+]
+
+# Runs ninja on /tmp/before and /tmp/after
+before-after = [
+  "compare",
+  "python3 build/gen.py --out-path=/tmp/$JJ_LABEL ${GN_OPTS:-} && ninja -C /tmp/$JJ_LABEL ${NINJA_TARGETS:-gn}",
+]
+
+compare-perf = [
+  "util",
+  "exec",
+  "--",
+  "bash",
+  "-c",
+  """
+  GN_OPTS="--gen=${SRC:-$HOME/chromium/src}=${OUT_DIR:-out/\\$JJ_LABEL}" \
+  NINJA_TARGETS=gen_bench \
+  jj --config "revset-aliases.before=\\"${BEFORE:-trunk()}\\"" before-after && \
+  python3 "$JJ_WORKSPACE_ROOT/tools/compare_traces.py" \
+    --before /tmp/before/gen_bench_[0-9]*.trace \
+    --after  /tmp/after/gen_bench_[0-9]*.trace
+  """,
+]
+
 [fix.tools.clang-format]
 command = ["clang/bin/clang-format", "--assume-filename=$path"]
 patterns = [