update changelog to say how many fallbacks we removed and how many remain
I just talked to teor about an idea where, when we say things like
- Remove broken entries from the hard-coded fallback directory list. Closes ticket 20190; patch by teor.
we should say how many we removed and how many remain.
This will be useful in the short term for reassuring ourselves that enough remain, and in the longer term for analyzing patterns in how our working fallback sets decay.