Only let karma scan for uncommitted files

It would be nice if karma provide config option like Jest to only scan for uncommitted files.
This will result in reduced delay.

I'm guessing karma current behavior is to scan source directory for files that match defined patterns. Imagine if there are 1000 files or more, this can take a really long time for browser to response, especially on low-end (before start testing), Wouldn't it be better if only some uncommitted files that need to be scan?

Start by locating Karma's configuration and source-directory file-scanning path, then compare Jest's uncommitted-file behavior. Clarify how the proposed option should select files and what browser startup should scan before defining completion criteria.

Written by the indexing model from the issue text.