Fix limit calculation for largeish Mercurial repsositories

Summary:
Fixes T10304. In Mercurial, we must enumerate the whole file tree. Currently, we incorrectly count files within directories (which won't be shown) toward the "100 file" limit at top level, so directories with more than 100 subpaths are truncated improperly.

This is approxiately the same as @richardvanvelzen's fix.

Test Plan: Viewed a large Mercurial repository, saw a complete directory listing.

Reviewers: chad

Reviewed By: chad

Subscribers: richardvanvelzen

Maniphest Tasks: T10304

Differential Revision: https://secure.phabricator.com/D15282