automate-git: Windows: Patch VS toolchain scripts before runhooks

Split `gclient sync` into separate `gclient sync --nohooks` and
`gclient runhooks` steps so that we can optionally apply patches
from `runhooks.patch` in-between. This is necessary because the
src/tools/clang/scripts/update.py script is run via a hook and has
dependencies on the VS toolchain scripts which must be patched to
properly consider the GYP_MSVS_VERSION environment variable.

patch/patches/runhooks.patch

@@ -0,0 +1,76 @@
+diff --git build/toolchain/win/setup_toolchain.py build/toolchain/win/setup_toolchain.py
+index fa31688f36a2..69d74aa31c47 100644
+--- build/toolchain/win/setup_toolchain.py
++++ build/toolchain/win/setup_toolchain.py
+@@ -142,22 +142,25 @@ def _LoadToolchainEnv(cpu, sdk_dir, target_store):
+       # variable.
+       if 'VSINSTALLDIR' in os.environ:
+         del os.environ['VSINSTALLDIR']
+-      other_path = os.path.normpath(os.path.join(
++      script_path = os.path.normpath(os.path.join(
+                                         os.environ['GYP_MSVS_OVERRIDE_PATH'],
+                                         'VC/Auxiliary/Build/vcvarsall.bat'))
+-      if not os.path.exists(other_path):
+-        raise Exception('%s is missing - make sure VC++ tools are installed.' %
+-                        script_path)
+-      script_path = other_path
+-    cpu_arg = "amd64"
+-    if (cpu != 'x64'):
+-      # x64 is default target CPU thus any other CPU requires a target set
+-      cpu_arg += '_' + cpu
+-    args = [script_path, cpu_arg]
+-    # Store target must come before any SDK version declaration
+-    if (target_store):
+-      args.append(['store'])
+-    variables = _LoadEnvFromBat(args)
++    if os.path.exists(script_path):
++      cpu_arg = "amd64"
++      if (cpu != 'x64'):
++        # x64 is default target CPU thus any other CPU requires a target set
++        cpu_arg += '_' + cpu
++      args = [script_path, cpu_arg]
++      # Store target must come before any SDK version declaration
++      if (target_store):
++        args.append(['store'])
++      variables = _LoadEnvFromBat(args)
++    else:
++      variables = []
++      for k in sorted(os.environ.keys()):
++        variables.append('%s=%s' % (str(k), str(os.environ[k])))
++      variables = '\n'.join(variables)
++
+   return _ExtractImportantEnvironment(variables)
+ 
+ 
+diff --git build/vs_toolchain.py build/vs_toolchain.py
+index 97600e73ba10..36773b159453 100755
+--- build/vs_toolchain.py
++++ build/vs_toolchain.py
+@@ -89,9 +89,16 @@ def SetEnvironmentAndGetRuntimeDllDirs():
+     runtime_path = os.path.pathsep.join(vs_runtime_dll_dirs)
+     os.environ['PATH'] = runtime_path + os.path.pathsep + os.environ['PATH']
+   elif sys.platform == 'win32' and not depot_tools_win_toolchain:
++    has_override_path = True
+     if not 'GYP_MSVS_OVERRIDE_PATH' in os.environ:
++      has_override_path = False
+       os.environ['GYP_MSVS_OVERRIDE_PATH'] = DetectVisualStudioPath()
+ 
++    if has_override_path:
++      # Don't attempt to copy DLLs when using a custom toolchain.
++      # The DLLs should already be discoverable via the PATH env variable.
++      return None
++
+     # When using an installed toolchain these files aren't needed in the output
+     # directory in order to run binaries locally, but they are needed in order
+     # to create isolates or the mini_installer. Copying them to the output
+@@ -140,6 +147,10 @@ def _RegistryGetValue(key, value):
+ def GetVisualStudioVersion():
+   """Return best available version of Visual Studio.
+   """
++  # Return the explicitly requested version, if any.
++  if 'GYP_MSVS_VERSION' in os.environ:
++    return os.environ['GYP_MSVS_VERSION']
++
+   supported_versions = list(MSVS_VERSIONS.keys())
+ 
+   # VS installed in depot_tools for Googlers