git_remote_helpers: fix input when running under Python 3

git_remote_helpers: fix input when running under Python 3

Although 2to3 will fix most issues in Python 2 code to make it run under
Python 3, it does not handle the new strict separation between byte
strings and unicode strings.  There is one instance in
git_remote_helpers where we are caught by this, which is when reading
refs from "git for-each-ref".

Fix this by operating on the returned string as a byte string rather
than a unicode string.  As this method is currently only used internally
by the class this does not affect code anywhere else.

Note that we cannot use byte strings in the source as the 'b' prefix is
not supported before Python 2.7 so in order to maintain compatibility
with the maximum range of Python versions we use an explicit call to
encode().

Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>