`file_only` region key is never read, dropping shared top-level code

[drewbitt]

extract_regions keys a side with no entities as file_only (region.rs:55). Nothing reads that key, so top-level text identical on all three sides is dropped. Exit 0, no warning, so git merge looks clean.

# base.ts      ours.ts       theirs.ts
let a=0        let a=0       b()
b()            let d=0
               b()

cp ours.ts A.ts && weave-driver base.ts A.ts theirs.ts 7 f.ts
# exit 0; A.ts is `let d=0`, b() gone

Statements, comments and strings all drop. reconstruct.rs:59 handles this shape but only for file_header, hence #99 fixed #95 and not this. Repros on 0.3.6, main (05b19bd), #113.