Fix printing error on git-rebase failure

Reverted commit 4511b131e6 had a bug where
squash_ret.success = True and empty_rebase = False. In that case, stdout
and stderr would already be string and trying to decode would fail.

Bug: 1071280
Change-Id: Iadcc526147ebb98aa7a91a7daa64ef44066e83b6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2163387
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
Auto-Submit: Josip Sokcevic <sokcevic@google.com>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>

git_common.py

@@ -662,7 +662,8 @@ def rebase(parent, start, branch, abort=False):
   except subprocess2.CalledProcessError as cpe:
     if abort:
       run_with_retcode('rebase', '--abort')  # ignore failure
-    return RebaseRet(False, cpe.stdout, cpe.stderr)
+    return RebaseRet(False, cpe.stdout.decode('utf-8', 'replace'),
+                     cpe.stderr.decode('utf-8', 'replace'))
 
 
 def remove_merge_base(branch):
@@ -766,7 +767,7 @@ def run_stream_with_retcode(*cmd, **kwargs):
     retcode = proc.wait()
     if retcode != 0:
       raise subprocess2.CalledProcessError(retcode, cmd, os.getcwd(),
-                                           None, None)
+                                           b'', b'')
 
 
 def run_with_stderr(*cmd, **kwargs):