[glean] Fix off-by-one error in Flow src.FileLines facts
[glean] Fix off-by-one error in Flow src.FileLines facts

Summary: There was a bug where `flow glean` was emitting `src.FileLines` facts that reported the first line of every file as 1 byte shorter than it actually was. This fixes that.

Reviewed By: donsbot

Differential Revision: D28966028

fbshipit-source-id: d147cf46a4a32436f5644b49e62aadbdb4862cae

mroch/flow,mroch/flow,facebook/flow,mroch/flow,facebook/flow,mroch/flow,mroch/flow,facebook/flow,mroch/flow,facebook/flow,facebook/flow,facebook/flow,mroch/flow,facebook/flow