_CalculateAddedDeps calls into depot tools' scm.py GetOldContents to
compare previous and current versions of the DEPS file. GetOldContents
attempts to 'git show <sha>:<path>'. The problem on Windows is that path
uses '\' but git show seems to want a posix path. Git show therefore
doesn't find the file and returns an empty output, leading presubmit to
think all the deps are new.

Bug:725933
Change-Id: Ifbbfbcba4be466d9be623826818fd191bd2ca525
Reviewed-on: https://chromium-review.googlesource.com/514142
Commit-Queue: Nodir Turakulov <nodir@chromium.org>
Reviewed-by: Nodir Turakulov <nodir@chromium.org>