title: How can I set a gdb source mapping for a relative path
body: <p>I have gdb attached to a process and just stopped at a function breakpoint.
  Unfortunately I can't see any source code since the mapping seems to be off.</p>
  
  <p>The file it looks for reports as: <code>../../qpy/QtCore/qpycore_chimera.cpp</code> and the downloaded source resides in <code>/home/user/debugging/pyqt5-5.4.2+dfsg/qpy/QtCore</code>.</p>
  
  <p>No matter what I set as source mapping it won't open the file. What would be the correct syntax for a relative source mapping?</p>