[sledge] Use map union rather than merge in depth-map join
[sledge] Use map union rather than merge in depth-map join

Summary:
Minor optimization, specializing `M.merge` in the join of depth maps to
`M.union`, which is slightly more efficient in the cases where it applies

Reviewed By: jberdine

Differential Revision: D39816306

fbshipit-source-id: a1e9e949f992f339302cba3a05827589011d5996

facebook/infer,facebook/infer,facebook/infer,facebook/infer,facebook/infer,facebook/infer,facebook/infer