Add gclient hook that checks local git configuration.

DEPS

 # This file is automatically processed to create .DEPS.git which is the file
 # that gclient uses under git.
 #
 # See http://code.google.com/p/chromium/wiki/UsingGit
 #
 # To test manually, run:
 #   python tools/deps2git/deps2git.py -o .DEPS.git -w <gclientdir>
 # where <gcliendir> is the absolute path to the directory containing the
 # .gclient file (the parent of "src").
 #
@@ skipping 797 matching lines @@
                 "--platform=win32",
                 "--directory",
                 "--recursive",
                 "--no_auth",
                 "--num_threads=16",
                 "--bucket", "chromium-apache-win32",
                 "src/third_party/apache-win32",
     ],
   },
   {
+    # Verify committers' ~/.netc, gclient and git are properly configured for
+    # write access to the git repo. To be removed sometime after Chrome to git
+    # migration completes (let's say Sep 1 2014). It will probably stay on HEAD
+    # of SVN history forever though.

[iannucci, 2014/08/19 00:42:29]

can drop the last sentence :)

[Vadim Sh., 2014/08/19 00:57:43]

Done.

+    "name": "check_git_config",
+    "pattern": ".",
+    "action": [
+        "python",
+        "src/tools/check_git_config.py",
+        "--running-as-hook",
+    ],
+  },
+  {
     # A change to a .gyp, .gypi, or to GYP itself should run the generator.
     "name": "gyp",
     "pattern": ".",
     "action": ["python", "src/build/gyp_chromium"],
   },
 ]