[py3] Return list of strings in GitChange.AllFiles

Currently, GitChange.AllFiles returns a list of byte[]. This is
inconsistent with py2 and may cause issues when byte[] is used when
string is expected.

Change-Id: I3e73bc5f71aa787f55357b8139cdeeabee056047
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2946934
Commit-Queue: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
Auto-Submit: Josip Sokcevic <sokcevic@google.com>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>

presubmit_support.py

@@ -1309,7 +1309,7 @@ class GitChange(Change):
     root = root or self.RepositoryRoot()
     return subprocess.check_output(
         ['git', '-c', 'core.quotePath=false', 'ls-files', '--', '.'],
-        cwd=root).splitlines()
+        cwd=root).decode('utf-8', 'ignore').splitlines()
 
 
 def ListRelevantPresubmitFiles(files, root):