Generalize python_git_runner for use in non-git scripts.

Retrying the change in https://codereview.chromium.org/1415993003/, with
fixes to properly handle being called with either Windows- or
POSIX-style paths (the previous attempt assumed it was always called
with POSIX-style paths under MSYS, which itself was attempting to fix
the assumption that it was always called with Windows-style paths, but
neither assumption was right).

This change doesn't actually use the wrapper in any new places, but
subsequent changes will be made to use it in places where
'exec python' is called directly, like gclient and fetch.

BUG=411518
R=iannucci@google.com, sky@google.com

Review URL: https://codereview.chromium.org/1411793006

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@297401 0039d316-1c4b-4281-b951-d872f2087c98

git-upstream-diff

@@ -6,4 +6,4 @@
 # git_upstream_diff.py -- Provide the diff between the current branch and its
 # upstream.
 
-. $(type -P python_git_runner.sh)
+. $(type -P python_runner.sh)