Fix off-by-one error in GHCi line reporting (Trac #10578)

Authored by osa1.

Description

Fix off-by-one error in GHCi line reporting (Trac Trac #10578)

Test Plan: I couldn't add tests because apparently line number
reporting was already working correctly when loading script files. I
don't know how to test by running commands using stdin, is this
supported?

Reviewers: austin, thomie, bgamari

Reviewed By: thomie, bgamari

Subscribers: hvr, thomie

Differential Revision: https://phabricator.haskell.org/D1067