git transfers are slow on macos

[ehuss]

Problem
Starting with nightly which includes #7939, I believe there is an issue where git fetches tend to be very slow on macos (repro on 10.14 and 10.15). I can't repro on Linux.

Fetching a relatively small repo like https://github.com/alexcrichton/bzip2-rs.git regularly takes about 1 minute for me, whereas previously it was about 2 seconds. It doesn't happen all the time, but the majority it does.

Attaching with a debugger it seemed to just be stuck in socket_read, however I have not investigated much.

Switching to Cargo's git2 curl implementation fixes the issue (it fetches in less than a second). This can be easily triggered by setting HTTP_TIMEOUT=60.

Steps

1. In a blank project, add a git dependency to https://github.com/alexcrichton/bzip2-rs.git.
2. cargo fetch
3. Takes about 1 minute to finish when it should be about 2 seconds.