These checks are preformed by libgit2 on checkout, but they're also useful for
performing checks in applications which do not involve checkout.

Expose them under `sys/` as it's still fairly in the weeds even for this
library.

The underlying code uses GIT_CONFIG_FILENAME_GLOBAL, which is .gitconfig.

Show more directly what the sorting modes correspond to in git's `rev-list` as
that's the reference implementation for what the possible sorting orders are.

Documentation fixes

Document giterr_last() use only after error. #4772