git: allow config retrieval as bool or int

git: allow config retrieval as bool or int

Allow retrieving configuration values as bool or int,
letting 'git config' normalize all the values instead
of trying to do it ourselves in bup.

Also use git config --null to avoid having to worry about
any whitespace issues, although .strip() on the output
would probably work just as well. Previously, the result
would include a terminating newline written out by the
git config command, which is not desirable.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Reviewed-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>