Use the function recently added to common in svndiff.py

GetModifiedFiles() function was added to common repo in
https://skia.googlesource.com/common/+/af1de867422c42855ae54ed3838c62ea445ea6b4

BUG=None
TEST=None
R=borenet@google.com

Review URL: https://codereview.chromium.org/634313006