pkg/exec: Allow multiple calls to Wait
pkg/exec: Allow multiple calls to Wait

This fixes a race in gitreceive where Wait is called after the
connection closes for cleanup as well as during the request lifecycle.
Sometimes the close notify is delivered just before the request
finishes resulting in two calls to Wait.

Signed-off-by: Jonathan Rudenberg <3692bfa45759a67d83aedf0045f6cb635a966abf@titanous.com>

lmars/flynn,lmars/flynn,flynn/flynn,flynn/flynn,lmars/flynn,flynn/flynn,lmars/flynn,flynn/flynn,flynn/flynn,lmars/flynn