Some `focus add` calls require subsequent `focus sync`

[arxanas]

Describe the bug

Some focus add calls seem to drop dependencies from previous focus sync calls, necessitating another focus sync. I'm not sure why this happens.

To Reproduce

See FIXME comment in the CI job at https://github.com/twitter/focus/pull/17/files#diff-21364b2e6fae1f2875cee1ab3daefb0685403687eaf8bc32b5c6eacda351c9d3R47-R48

Expected behavior

The focus sync call should not be necessary.

[bierbaum]

I think this may be fixed since we corrected the mutate method's backing up of the sparse profile. Let's try to see if we can repro it now.