Don't let Diffusion show that an importing repository is "100%" imported

Summary:
A few users have hit this and found it confusing. Currently, it means "more than 99.95%", which is very different from "100%". Instead:

• show an extra digit of precision; and
• cap the display at "99.99%", so it's more clear that work is still happening.

Test Plan: Faked it and saw it cap at 99.99%.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D8058