GN: re-run echo_headers if git state changes.

BUILD.gn

 # Copyright 2016 Google Inc.
 #
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 declare_args() {
 }
 
 skia_public_includes = [
   "include/android",
@@ skipping 271 matching lines @@
 
   if (is_fuchsia) {
     sources += [ "src/ports/SkFontMgr_empty_factory.cpp" ]
   }
 }
 
 action("skia.h") {
   script = "gn/echo_headers.py"
   args = [ rebase_path("$target_gen_dir/skia.h", root_build_dir) ] +
          rebase_path(skia_public_includes, root_build_dir)
+  inputs = [ ".git/logs/HEAD" ]   # ~~> Run any time GIT state changes.
   outputs = [
     "$target_gen_dir/skia.h",
   ]
 }
 
 executable("fiddle") {
   include_dirs = [ "$target_gen_dir" ]
   libs = []
   if (is_linux) {
     libs += [ "OSMesa" ]
@@ skipping 180 matching lines @@
       ":flags",
       ":gm",
       ":gpu_tool_utils",
       ":skia",
       ":tool_utils",
       "//third_party/jsoncpp",
     ]
     testonly = true
   }
 }