[WriteTransaction] Avoid growing m_pendingOperations twice on replace

Summary:
As the following addTerms() does another reserve() with growing by
terms.size() sum up both sizes. The reserve() in addTerms() is a
noop then.

Reviewers: Baloo, ngraham

Reviewed By: Baloo, ngraham

Subscribers: kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D27771