[infra] Add a GN flag to omit git information

After Clang 6.0, changing the value of a single string can change the
order of symbols in the .rodata section. This confuses Golem's
checks for the equivalence of two binaries.

This change adds a GN flag to omit the git hash and commit time from
the Dart VM's version string to avoid re-running Golem benchmarks on
commits where the only VM change is in the version string.

To use it, pass --gn-args="dart_version_git_info=false" to gn.py

Change-Id: I8134714d50e3c9b6dae750a515dc3c2228d76b53
Reviewed-on: https://dart-review.googlesource.com/33000
Reviewed-by: Siva Annamalai <asiva@google.com>
Commit-Queue: Zach Anderson <zra@google.com>

runtime/BUILD.gn

@@ -339,4 +339,7 @@ action("generate_version_cc_file") {
     "--input",
     rebase_path("vm/version_in.cc", root_build_dir),
   ]
+  if (!dart_version_git_info) {
+    args += [ "--ignore_svn_revision" ]
+  }
 }

runtime/runtime_args.gni

@@ -52,4 +52,8 @@ declare_args() {
   # core-jit snapshot breaks the ability to change various flags that affect
   # code generation.
   dart_core_snapshot_kind = "core"
+
+  # Whether the Dart binary version string should include the git hash and
+  # git commit time.
+  dart_version_git_info = true
 }

tools/make_version.py

@@ -62,7 +62,7 @@ def makeFile(quiet, output_file, input_file, ignore_svn_revision):
   version_cc_text = version_cc_text.replace("{{VERSION_STR}}",
                                             version_string)
   version_time = utils.GetGitTimestamp()
-  if version_time == None:
+  if ignore_svn_revision or version_time == None:
     version_time = "Unknown timestamp"
   version_cc_text = version_cc_text.replace("{{COMMIT_TIME}}",
                                             version_time)