Git pull stuck when Trace2 target set to Unix Stream Socket

[Son Luong Ngoc <sluongng@gmail.com>]

Hey folks,

I am trying to write a simple git trace2 event collector and I notice
that when git doing git pull with trace events being sent to a unix
stream socket, the entire operation halted.

Reproduce as follow:
```
cd git/git
git config trace2.eventTarget af_unix:stream:/tmp/git_trace.sock
git config trace2.eventBrief false
(rm /tmp/git_trace.sock | ) &&  nc -lkU /tmp/git_trace.sock

# In a different terminal
git pull # Pull stuck and never complete
```

This does not happen when you set eventBrief to true
```
git config trace2.eventBrief true
```

Worth to note that if eventTarget is a file instead of a socket,
everything works fine.

Cheers,
Son Luong.