mail.sh: eliminate dependency on Git 1.8.5 or later
Since
95aca0f365 (mail.sh: various minor updates, 2016-06-03),
mail.sh has started using the --batch-check option. For efficiency
it has been passing in a format (it's slightly faster if you do not
ask for object length if you don't need it).
However, since the output fields mail.sh needs are part of the
default format for --batch-check and --batch-check without any format
has been supported since version 1.5.6 it's easy to make do without
the format. Therefore suppress the format unless running at least
Git version 1.8.5.
Signed-off-by: Kyle J. McKay <mackyle@gmail.com>