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

python_git_runner.sh

+# 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]/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 git_foo_command.py`
+
+## Constants
+PYTHONDONTWRITEBYTECODE=1
+
+## "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
+
+if [[ $OSTYPE = msys ]]
+then
+  DEPOT_TOOLS="${0%\\*}"
+  BASENAME="${0##*\\}"
+else
+  DEPOT_TOOLS="${0%/*}"
+  BASENAME="${0##*/}"
+fi
+SCRIPT="${SCRIPT-${BASENAME//-/_}.py}"
+
+if [[ -e "$DEPOT_TOOLS/python.bat" && $OSTYPE = msys ]]; then
+  cmd.exe //c "$DEPOT_TOOLS\\python.bat" "$DEPOT_TOOLS\\$SCRIPT" "$@"
+else
+  exec "$DEPOT_TOOLS/$SCRIPT" "$@"
+fi