When not sending data to a subprocess, close its stdin

We were letting stdin untouched, which meant child processes had
access to our terminal input. That meant `!fmt` was trying to read
from our terminal input and catching keystrokes.

Fixes #1281

src/shell_manager.cc

@@ -171,6 +171,8 @@ std::pair<String, int> ShellManager::eval(
             close(child_stdin.write_fd());
             move(child_stdin.read_fd(), 0);
         }
+        else
+            close(0);
 
         close(child_stdout.read_fd());
         move(child_stdout.write_fd(), 1);