allow v6 ninja log format

This is for https://crrev.com/c/5632535.
v6 in 1.12.1 is the same format with v5 in 1.11.1 actually.
ref: https://github.com/ninja-build/ninja/pull/2240


Bug: 346910589
Change-Id: Id13fc0945d1d7f80b42b87c37969c01ffa113e24
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5633274
Reviewed-by: Junji Watanabe <jwata@google.com>
Commit-Queue: Junji Watanabe <jwata@google.com>
Auto-Submit: Takuto Ikuta <tikuta@chromium.org>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>

post_build_ninja_summary.py

@@ -121,8 +121,8 @@ def ReadTargets(log, show_all):
     # targets.
     if not header:
         return []
-    assert header == "# ninja log v5\n", ("unrecognized ninja log version %r" %
-                                          header)
+    assert header in ("# ninja log v5\n", "# ninja log v6\n"), (
+        "unrecognized ninja log version %r" % header)
     targets_dict = {}
     last_end_seen = 0.0
     for line in log: