Fix a bug with DiffusionMergedCommitsQueryConduitAPIMethod

Summary: We were adding "+1" to PHP_INT_MAX, which could cause us to wrap and run git log -n -9223372036854775808 .... Git, unable to give us negative nine trillion zillion results, would then exit without producing results.

Test Plan: Got results out of this method.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D12726