git: add `--super-prefix` to usage string

git: add `--super-prefix` to usage string

When the `--super-prefix` option was implmented in 74866d7579 (git: make
super-prefix option, 2016-10-07), its existence was only documented in
the manpage but not in the command's own usage string. Given that the
commit message didn't mention that this was done intentionally and given
that it's documented in the manpage, this seems like an oversight.

Add it to the usage string to fix the inconsistency.

Signed-off-by: Patrick Steinhardt <redacted>
Signed-off-by: Junio C Hamano <redacted>