Fixing forall subcommand for Py3

Execution of 'repo forall -p -c' doesn't work with Py3 and ends up
with an error:

Got an error, terminating the pool: TypeError: can only concatenate
str (not "bytes") to str

That's fixed by using the decode() method.

Change-Id: Ice01aaa1822dde8d957b5bf096021dd5a2b7dd51
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/253659


Reviewed-by: Mike Frysinger <vapier@google.com>
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Jiri Tyr <jiri.tyr@gmail.com>
(cherry picked from commit 83a3227b)