For fetches over HTTP, Workhorse executes git-upload-pack and first
attempts to send all the input data to stdin before reading from the
stdout pipe. However, when the payload is large, the stdout pipe may
fill up, causing git-upload-pack to stop reading from stdin. Workhorse
will then be deadlocked, since it will be waiting to send more data
to a buffer that will never be drained.

An addition side effect is that git-upload-pack processes also get left
around. These processes are cleaned up only after Workhorse is
restarted.

This fix modifies the git-upload-pack behavior to consume the entire
HTTP input first so that reading the data from stdout and sending the
reply can be performed in a separate Goroutine.

Closes #92

Closes gitlab-org/gitlab-ce#25916

Closes gitlab-com/infrastructure#941