t3700: merge two tests into one

t3700: merge two tests into one

Depending on the underlying platform a chmod may be a noop. Although it
wouldn't harm the result of the '--chmod=-x' test, there is a more
robust way to make sure the --chmod option works both ways.

Merge the two separate tests for the --chmod option into one, checking
both permissions on the same file.

Signed-off-by: Ingo Brückl <ib@wupperonline.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>