parse-options: rename `OPT_MAGNITUDE()` to `OPT_UNSIGNED()`

parse-options: rename `OPT_MAGNITUDE()` to `OPT_UNSIGNED()`

With the preceding commit, `OPT_INTEGER()` has learned to support unit
factors. Consequently, the major differencen between `OPT_INTEGER()` and
`OPT_MAGNITUDE()` isn't the support of unit factors anymore, as both of
them do support them now. Instead, the difference is that one handles
signed and the other handles unsigned integers.

Adapt the name of `OPT_MAGNITUDE()` accordingly by renaming it to
`OPT_UNSIGNED()`.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>