Add GetModifiedFiles() function to git_utils.

It will be used in a follow up CL to replace _GitGetModifiedFiles() from
tools/svndiff.py in skia.googlesource.com/skia repo.

BUG=None
TEST=./py/run_unittests
R=borenet@google.com

Review URL: https://codereview.chromium.org/638723002

py/utils/git_utils.py

diff --git a/py/utils/git_utils.py b/py/utils/git_utils.py
index fec79d7..e0bb6e0 100644
--- a/py/utils/git_utils.py
+++ b/py/utils/git_utils.py
@@ -89,6 +89,12 @@
                           'refs/heads/master']).rstrip()
 
 
+def GetModifiedFiles():
+  """Returns a list of locally modified files within the current working dir.
+  """
+  return shell_utils.run([GIT, 'ls-files', '-m']).splitlines()
+
+
 def GetCurrentBranch():
   return shell_utils.run([GIT, 'rev-parse', '--abbrev-ref', 'HEAD']).rstrip()