Since commit 61495cdaae (Fix Xcode project references to the source
tree, 2009-09-22, v2.8.0~43) we force source file references to use
relative paths from the source tree. If the source tree path is a
symbolic link inside the build tree, the relative `../` sequence
goes to the wrong place. The problem with debug breakpoints motivating
that change does not seem to occur in modern Xcode versions, so update
the logic to use a relative path only when it does not need to start
in any `../` sequence.
de766bc7e0