This is a reland of 01ed3589

* python_runner.sh, gclient, roll-dep and fetch will call vpython
  (instead of vpython3) when running on Windows under git-bash.
* vpython3 now detects when running on Windows under git-bash and
  executes vpython3.bat instead.
* vpython3.bat calls python3.exe directly instead of calling python3.bat.

Original change's description:
> depot_tools: Run using Python 3 by default.
>
> Run gclient, roll-dep, fetch and custom git commands (i.e. git-cl, git-rebase-update, git-new-branch, etc.)
> using vpython3 by default.
>
> Change-Id: I4eecddafa6ca4c5f82ec097615c79d2a741613e7
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2113550
> Reviewed-by: Anthony Polito <apolito@google.com>
> Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

Change-Id: I9829141d7ea26a67e655264430151f493e73a930
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2118418
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Anthony Polito <apolito@google.com>