fix(tmux): Handle another error we might get when listing sessions.

I was already handling one situation where the server isn't running, but
this situation is if the tmux socket doesn't yet exist.

git/git/git.go

@@ -116,7 +116,7 @@ func (repo *gitRepo) parseOriginURL() string {
 		if strings.Contains(stderr, "No such remote") {
 			return ""
 		}
-		fmt.Fprint(os.Stderr, stderr)
+		fmt.Fprintln(os.Stderr, stderr)
 		return ""
 	}
 	for _, regex := range urlRegexes {
@@ -184,7 +184,7 @@ func (repo *gitRepo) configValue(key string) (string, error) {
 		if stderr == "" {
 			return "", nil
 		}
-		fmt.Fprint(os.Stderr, stderr)
+		fmt.Fprintln(os.Stderr, stderr)
 		return "", err
 	}
 	return stdout, nil

git/git/git_test.go

@@ -246,7 +246,7 @@ func (cmd repoCommand) Run(repo *gitRepo) error {
 	orig := repoCmd.Stderr
 	stderr, _ := repoCmd.RunStderr()
 	if !strings.Contains(stderr, cmd.errorMessage) {
-		fmt.Fprint(orig, stderr)
+		fmt.Fprintln(orig, stderr)
 		return fmt.Errorf("did not observe expected error message containing %q", cmd.errorMessage)
 	}
 	return nil

tmux/server.go

@@ -145,10 +145,14 @@ func (srv *Server) Equal(other *Server) bool {
 func (srv *Server) ListSessions() ([]*Session, error) {
 	stdout, stderr, err := srv.command("list-sessions", "-F", string(SessionID)).RunOutput()
 	if err != nil {
-		if strings.Contains(stderr, "no server running") {
+		if
+		// Socket doesn't yet exists.
+		strings.Contains(stderr, "No such file or directory") ||
+			// Socket exists, but no server owns it.
+			strings.Contains(stderr, "no server running") {
 			return nil, nil
 		}
-		fmt.Fprint(os.Stderr, stderr)
+		fmt.Fprintln(os.Stderr, stderr)
 		return nil, err
 	}
 	var res []*Session