[lib] Misc naming tweaks in `mergeNewMessages`
[lib] Misc naming tweaks in `mergeNewMessages`

Summary: Thought these changes made things a little more readable. No functional changes.

Test Plan: should be straightforward refactor

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: palys-swm, Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D1933

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