Adjust line offsets when the current line was not changed since the last LSIF index

Created by: efritz

Currently, a user browsing commit c which is not indexed will get precise code intelligence via a nearest index at (for example) commit c-n. If there are changes to this file between these commits, then the user may be requesting a range that is not equivalent between the two commits. (Specifically, if there are additions before it then the range's offset is too high; if there are deletions then it's too low).

See for a recent example of this behavior.

Instead, we should be adjusting the line offsets before querying the LSIF backend to be the offset in the indexed commit. Otherwise we're getting precise intelligence for random symbols that happen to be on the same line in a commit distinct from the one being viewed.