Throw an error when get() is passed no section.

Reported in theory/sqitch#110. It was doing this:

    t/sqitch config --get user
    Use of uninitialized value $section in lc at lib/Config/GitLike.pm line 658.

But that is not what Git does:

    > git config --get user
    error: key does not contain a section: user

This patch makes Config::GitLike behave a bit more like Git in this respect.