t7406: fix breakage on OSX

t7406: fix breakage on OSX

On OSX `wc` prefixes the output of numbers with whitespace, such
that the `commit_count` would be "SP <NUMBER>". When using that in

    git submodule update --init --depth=$commit_count

the depth would be empty and the number is interpreted as the
pathspec.  Fix this by not using `wc` and rather instruct rev-list
to count.

Another way to fix this is to remove the `=` sign after the
`--depth` argument as then we are allowed to have more than just one
whitespace between `--depth` and the actual number.  Prefer the
solution of rev-list counting as that is expected to be slightly
faster and more self-contained within Git.

Reported-by: Lars Schneider <larsxschneider@gmail.com>
Helped-by: Junio C Hamano <gitster@pobox.com>,
Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>