Git Space Quota Usage

**Describe the bug**
I've begun getting notices from the app that my Git usage is above 100GB. When I check the total of all my Keybase repositories (e.g., with git-sizer), however, I don't see that they're using more than perhaps 20–40GB. Consequently, it's difficult to tell whether (1) Keybase is simply reporting my quota usage erroneously or (2) there is indeed one or more repositories occupying more space than shown when I try to assess their sizes. I've totally deleted and restarted the largest repository, which looks like it should have freed about 25GB of Git storage quota. In the interim, none of the other repositories has grown by this amount, but I'm still getting the notice.

**To Reproduce**
Steps to reproduce the behavior:
1. Use Keybase Git for a while, including with larger repositories.
2. Get surprised when what looks like the amount of space the repositories are using isn't actually what the Keybase client reports.

**Expected behavior**
A clear account from the Keybase client of which repositories it thinks are using how much space, and clear reductions in total usage when larger repositories are eliminated to reclaim storage quota.

**Screenshots**

**Additional numbers**
The number 345567

No source files, tests, or entry points are named in the issue. Start by reproducing the quota discrepancy with the Keybase client and git-sizer, then trace how repository usage is reported; done means the client identifies per-repository usage and reflects deleted repositories in the total quota.

Written by the indexing model from the issue text.