Refactor the way that git executables are launched in depot tools.

python_git_runner.sh

+#!/bin/bash
+# Copyright (c) 2014 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+## This file is designed to be sourced from a bash script whose name takes the
+## form 'git-sub-command'. This script will then instead invoke
+## '[depot_tools]/gitscripts/git_sub_command.py' correctly under mingw as well
+## as posix-ey systems, passing along all other command line flags.
+
+## Example:
+## echo ". python_git_runner.sh" > git-foo-command
+## ./git-foo-command  #=> runs `python gitscripts/git_foo_command.py`
+
+## Constants
+PYTHONDONTWRITEBYTECODE=1
+SUBDIR=gitscripts
+BASE="$(basename "$0")"
+
+## "Input parameters".
+# If set before the script is sourced, then we'll use the pre-set values.
+# SCRIPT defaults to the basename of $0, with dashes replaced with underscores
+SCRIPT="${SCRIPT-${BASE//-/_}.py}"
+
+OUTPUT="$(uname | grep 'MINGW')"
+MINGW=$?
+
+if [ $MINGW = 0 ]; then
+  base_dir="${0%\\*}"
+else
+  base_dir=$(dirname "$0")
+fi
+
+if [ -e "$base_dir/python.bat" -a $MINGW = 0 ]; then
+  cmd.exe //c "$base_dir\\python.bat" "$base_dir\\$SUBDIR\\$SCRIPT" "$@"
+else
+  exec "$base_dir/$SUBDIR/$SCRIPT" "$@"
+fi