[build] Automatically rebuild sdk_hash when the Git commit changes.

Compare the dependencies of version.cc.

Bug: https://github.com/dart-lang/sdk/issues/42633
Change-Id: I3ad50736fa73cbba2c2a5a582cbf1c76e49c35f8
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/153860
Commit-Queue: Ryan Macnak <rmacnak@google.com>
Reviewed-by: Alexander Aprelev <aam@google.com>

sdk_args.gni

@@ -34,6 +34,7 @@ if (verify_sdk_hash) {
                          [
                            "$_dart_root/tools/VERSION",
                            "$_dart_root/tools/utils.py",
+                           "$default_git_folder/logs/HEAD",
                          ])
 } else {
   sdk_hash = "0000000000"