Batch execution of LastModified query

Summary:
Ref T2683. Further reduces query count of last modified loads; we're now at 11 instead of 200+.

(This works in SVN but could be further optimized.)

Test Plan:
Loaded SVN, Mercurial, Git:

{F34864}
{F34865}
{F34866}

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley, vrana, aran

Maniphest Tasks: T2683

Differential Revision: https://secure.phabricator.com/D5256