depot_tools: Run git_common_test in Python 3

Bug: 1009809
Change-Id: Idfcbd26de3420798f092c7fa55a6126d7c389a8c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1834317
Reviewed-by: Anthony Polito <apolito@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
diff --git a/git_number.py b/git_number.py
index f3d2da7..0e3ac16 100755
--- a/git_number.py
+++ b/git_number.py
@@ -75,7 +75,8 @@
   ref = '%s:%s' % (REF, pathlify(prefix_bytes))
 
   try:
-    raw = buffer(git.run('cat-file', 'blob', ref, autostrip=False))
+    raw = buffer(
+        git.run('cat-file', 'blob', ref, autostrip=False, decode=False))
     return dict(struct.unpack_from(CHUNK_FMT, raw, i * CHUNK_SIZE)
                 for i in xrange(len(raw) / CHUNK_SIZE))
   except subprocess2.CalledProcessError: