Correct Git show's path format on Windows

_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>

scm.py

@@ -8,6 +8,7 @@ import cStringIO
 import glob
 import logging
 import os
+import platform
 import re
 import sys
 import tempfile
@@ -258,6 +259,9 @@ class GIT(object):
   def GetOldContents(cwd, filename, branch=None):
     if not branch:
       branch = GIT.GetUpstreamBranch(cwd)
+    if platform.system() == 'Windows':
+      # git show <sha>:<path> wants a posix path.
+      filename = filename.replace('\\', '/')
     command = ['show', '%s:%s' % (branch, filename)]
     try:
       return GIT.Capture(command, cwd=cwd, strip_out=False)

tests/scm_unittest.py

@@ -58,6 +58,7 @@ class RootTestCase(BaseSCMTestCase):
         'logging',
         'only_int',
         'os',
+        'platform',
         're',
         'subprocess2',
         'sys',