[build] Don't depend on the Git repo using --ref-format=files.

With the newer --ref-format=reftable, .git/logs/HEAD does not exist, so the version generation steps would always be considered dirty.

Cf. c7e58f832a

Change-Id: Id8735a739b751e4a54c2e4beecbef1eb23771c53
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/488600
Commit-Queue: Brian Quinlan <bquinlan@google.com>
Reviewed-by: Brian Quinlan <bquinlan@google.com>
Commit-Queue: Ryan Macnak <rmacnak@google.com>

utils/BUILD.gn

@@ -57,3 +57,9 @@ aot_compile_using_prebuilt_sdk("gen_kernel.exe") {
   output = "$root_out_dir/gen_kernel.exe"
   package_config = "$_dart_root/.dart_tool/package_config.json"
 }
+
+# Does not write to the file if the content is unchanged.
+generated_file("git_version") {
+  outputs = [ "$target_gen_dir/git_version" ]
+  contents = exec_script("../tools/make_version.py", [], "trim string")
+}