GN tool: copy_git_directory.py script

Motivation: to be used by android roller (android_skia_checkout.go)
to keep third_party/libgif updated to where DEPS points
third_party/external/libgif at.

No-Try: true
Bug: skia:9654
Change-Id: I18273dfc7da215b6081ffd454edfa1651791b6df
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/254896
Commit-Queue: Hal Canary <halcanary@google.com>
Reviewed-by: Leon Scroggins <scroggo@google.com>

gn/copy_git_directory.py

@@ -0,0 +1,59 @@
+#! /usr/bin/env python
+# Copyright 2019 Google LLC.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+import os
+import shutil
+import subprocess
+import sys
+
+def copy_git_directory(src, dst, out=None):
+  '''
+  Makes a copy of `src` directory in `dst` directory.  If files already exist
+  and are identical, do not touch them.  If extra files or directories exist,
+  remove them.  Assume that `src` is a git directory so that `git ls-files` can
+  be used to enumerate files.  This has the added benefit of ignoring files
+  not tracked by git.  Also, if out is not None, write summary of actions to out.
+  If `dst` is a top-level git directory, the `.git` directory will be removed.
+  '''
+  assert os.path.isdir(src)
+  if not os.path.isdir(dst):
+    os.makedirs(dst)
+  ls_files = subprocess.check_output(['git', 'ls-files', '-z', '.'], cwd=src)
+  src_files = set(p for p in ls_files.split('\0') if p)
+  abs_src = os.path.abspath(src)
+  cwd = os.getcwd()
+  try:
+    os.chdir(dst)
+    def output(out, sym, dst, path):
+      if out:
+        out.write('%s %s%s%s\n' % (sym, dst, os.sep, path))
+    for dirpath, dirnames, filenames in os.walk('.', topdown=False):
+      for filename in filenames:
+        path = os.path.normpath(os.path.join(dirpath, filename))
+        if path not in src_files:
+          output(out, '-', dst, path)
+          os.remove(path)
+      for filename in dirnames:
+        path = os.path.normpath(os.path.join(dirpath, filename))
+        if not os.listdir(path):  # Remove empty subfolders.
+          output(out, '-', dst, path + os.sep)
+          os.rmdir(path)
+    for path in src_files:
+      src_path = os.path.join(abs_src, path)
+      if os.path.exists(path):
+        with open(path) as f1:
+          with open(src_path) as f2:
+            if f1.read() == f2.read():
+              continue
+      output(out, '+', dst, path)
+      shutil.copy2(src_path, path)
+  finally:
+    os.chdir(cwd)
+
+if __name__ == '__main__':
+  if len(sys.argv) != 3:
+    sys.stderr.write('\nUsage:\n  %s SRC_DIR DST_DIR\n\n' % sys.argv[0])
+    sys.exit(1)
+  copy_git_directory(sys.argv[1], sys.argv[2], sys.stdout)