ClangFormat: report a relative path rather than an absolute one.
ClangFormat: report a relative path rather than an absolute one.

Indeed, to have an absolute path may not be very useful, especially
when it comes from Buildbot.

nickerso/libcellml,hsorby/libcellml,MichaelClerx/libcellml,nickerso/libcellml,cellml/libcellml,cellml/libcellml,nickerso/libcellml,hsorby/libcellml,nickerso/libcellml,MichaelClerx/libcellml,cellml/libcellml,MichaelClerx/libcellml,hsorby/libcellml,hsorby/libcellml,cellml/libcellml