This CL relaxes the check for the .git folder on Windows so that it
may be either a file or a folder, as may be the case if depot_tools
is a git submodule or a worktree.

Bug: 1333997
Change-Id: Ib805912903696fc247f85d15c35b6e3093cd57f9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3736684
Commit-Queue: Will Cassella <cassew@chromium.org>
Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Reviewed-by: Aravind Vasudevan <aravindvasudev@google.com>