parse-options: support unit factors in `OPT_INTEGER()`

There are two main differences between `OPT_INTEGER()` and
`OPT_MAGNITUDE()`:

- The former parses signed integers whereas the latter parses unsigned
integers.

- The latter parses unit factors like 'k', 'm' or 'g'.

While the first difference makes obvious sense, there isn't really a
good reason why signed integers shouldn't support unit factors, too.

This inconsistency will also become a bit of a problem with subsequent
commits, where we will fix a couple of callsites that pass an unsigned
integer to `OPT_INTEGER()`. There are three options:

- We could adapt those users to instead pass a signed integer, but
this would needlessly extend the range of accepted integer values.

- We could convert them to use `OPT_MAGNITUDE()`, as it only accepts
unsigned integers. But now we have the inconsistency that we also
start to accept unit factors.

- We could introduce `OPT_UNSIGNED()` as equivalent to `OPT_INTEGER()`
so that it knows to only accept unsigned integers without unit
suffix.

Introducing a whole new option type feels a bit excessive. There also
isn't really a good reason why `OPT_INTEGER()` cannot be extended to
also accept unit factors: all valid values passed to such options cannot
have a unit factors right now, so there wouldn't be any ambiguity.

Refactor `OPT_INTEGER()` to use `git_parse_int()`, which knows to
interpret unit factors. This removes the inconsistency between the
signed and unsigned options so that we can easily fix up callsites that
pass the wrong integer type right now.

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