Generalize python_git_runner for use in non-git scripts.

python_runner.sh

-# Copyright 2014 The Chromium Authors. All rights reserved.
+# Copyright 2015 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
+## form 'command-name'. This script will then instead invoke
+## '[depot_tools]/command_name.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
+## echo ". python_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="${0##*/}"
+DEPOT_TOOLS="${0%/*}"
+# Sometimes commands will run with no path (e.g. a git command run from within
+# the depot_tools dir itself). In that case, treat it like it was run like:
+# "./command"
+if [[ "$DEPOT_TOOLS" = "$0" ]]; then
+  DEPOT_TOOLS="."
 fi
+BASENAME="${0##*/}"
 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