R1:f7464400a510

Limit memory usage of `ssh-exec` during large pull operations

Summary: Fixes T4241. Ref T4206. See T4241 for a description here. Generally, when we connect a fat pipe (`git-upload-pack`) to a narrow one (`git` over SSH) we currently read limitless data into memory. Instead, throttle reads until writes catch up. This is now possible because of the previous changes in this sequence.

Test Plan:
- Ran `git clone` and `git push` on the entire Wine repository.
- Observed CPU and memory usage.
- Memory…