Fix GetOldContents() to return an empty string if the file didn't exist.

Bug: 700108
Change-Id: I9010aa3b4aa467c91b93ffac924b3ec5a16cfaa2
Reviewed-on: https://chromium-review.googlesource.com/476590
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>

scm.py

@@ -259,7 +259,10 @@ class GIT(object):
     if not branch:
       branch = GIT.GetUpstreamBranch(cwd)
     command = ['show', '%s:%s' % (branch, filename)]
+    try:
       return GIT.Capture(command, cwd=cwd, strip_out=False)
+    except subprocess2.CalledProcessError:
+      return ''
 
   @staticmethod
   def GenerateDiff(cwd, branch=None, branch_head='HEAD', full_move=False,