…ndex

This doesn't happen here though, so nothing special we could trigger.

That way, it's possible to not compile a bunch of code in `gix` if
the writing of packs isn't required.

This should make optimizing compile time and performance easier, while
assuring these options aren't pre-determined by library providers.

This also removes all diff capabilities.

That way users can more precisely decide what they want to use.
Note that spec-parsing is so foundational that it's always included.
Those who don't need it nor need describe don't need the crate in the
fist place.

`revparse-regex` is only used when parsing revspecs that use a special syntax.
This feature is also enabled by default.

This makes it easier to use.

…feature toggle.

They are also available when using https transports.

Even though there is a chance that overall, it will compile multiple
different versions of the same thing so it's not actually faster.

But let's not think too much about that.