Generalize python_git_runner for use in non-git scripts.

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

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=297401

[Michael Moss, 2015-10-29 16:47:03]

mmoss@chromium.org changed reviewers:
+ estaab@chromium.org

[Michael Moss, 2015-10-29 16:47:04]

+estaab as trooper

[estaab, 2015-10-29 16:47:36]

lgtm

[Michael Moss, 2015-10-29 16:50:31]

The CQ bit was checked by mmoss@chromium.org

[commit-bot: I haz the power, 2015-10-29 16:50:42]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1411793006/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1411793006/1

[commit-bot: I haz the power, 2015-10-29 16:55:39]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-10-29 16:55:39]

Try jobs failed on following builders:
  depot_tools_presubmit on tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/depot_tools_pre...)

[Michael Moss, 2015-10-29 16:56:52]

Ugh, still needs OWNER LGTM.

[iannucci, 2015-10-30 01:13:17]

iannucci@chromium.org changed reviewers:
+ iannucci@chromium.org

[iannucci, 2015-10-30 01:13:18]

scary :(

lgtm

[Michael Moss, 2015-10-30 15:37:13]

The CQ bit was checked by mmoss@chromium.org

[commit-bot: I haz the power, 2015-10-30 15:37:30]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1411793006/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1411793006/1

[commit-bot: I haz the power, 2015-10-30 15:39:26]

Committed patchset #1 (id:1) as
http://src.chromium.org/viewvc/chrome?view=rev&revision=297401