wp_dashboard_recent_comments function: avoid unnecessary SQL query

[kalessil]

Tune wp_dashboard_recent_comments performance:

• further reduce the number of count calls (related to the history of changes in this function)
• harden while-loop condition to avoid SQLs when the last query fetched less than what was aimed (next query fetches nothing)

Trac ticket: https://core.trac.wordpress.org/ticket/64506

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