Ensure the port isn't open before our git daemon

If a previous run of the git daemon still has port 9418 open, then
we might think we successfully ran `git daemon`, when instead our
asynchronous command could be failing to bind the port and quitting
immediately. (The use of `set -e` would not mitigate this, since
the `git daemon ... &` command would have successfully forked off
the asynchronous subprocess.)

One way that could happen, at least in principle, is if the `EXIT`
trap from a previous run did not cause the `git-daemon` process to
terminate. That could happen either due to:

- The `EXIT` trap somehow not running.

- The child `git-daemon` process not being caused to terminate when
  its parent `git` process terminates. They are actually separate
  processes: Running a `git daemon ...` command creates a `git`
  process, which then creates a `git-daemon` subprocess (where
  `git-daemon` is in the `git-core` directory `git --exec-path`
  gives). The parent `git` process forks before exec-ing the child
  `git-daemon` process. So they have separate PIDs. The PID from
  `$!` belongs to the parent `git` process.

  However, both processes are usually terminated, at least in
  simplified local experiments.

This change aims to detect such conditions. (It might be worth
keeping even if that kind of problem is not currently happening.)

tests/helpers.sh

@@ -60,9 +60,14 @@ function small-repo-in-sandbox() {
 }
 
 function launch-git-daemon() {
+    local port=9418
+    if nc -z localhost "$port"; then
+      echo "Port $port should not have been open before this test's run of the git daemon!" >&2
+      return 1
+    fi
     git -c uploadpack.allowrefinwant daemon --verbose --base-path=. --export-all --user-path &>/dev/null &
     daemon_pid=$!
-    while ! nc -z localhost 9418; do
+    while ! nc -z localhost "$port"; do
       sleep 0.1
     done
     trap 'kill $daemon_pid' EXIT