No longer preallocate repo slice in repohasfile filtering

Created by: ijt

This follows up on a comment by @keegancsmith on a previous PR: https://github.com/sourcegraph/sourcegraph/pull/5189#discussion_r314038793, where he explained that preallocating is probably less memory efficient in the typical case for the line of code in question because usually the output slice is much smaller.

Test plan: relying on existing tests