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>

subcmds/forall.py

@@ -366,7 +366,7 @@ def DoWork(project, mirror, opt, cmd, shell, cnt, config):
     while not s_in.is_done:
       in_ready = s_in.select()
       for s in in_ready:
-        buf = s.read()
+        buf = s.read().decode()
         if not buf:
           s.close()
           s_in.remove(s)