Fixing Debian bug 689310 about opening files from command line.

If the filename is passed as relative (e.g. "kbibtex test.bib"),
the file is not opened. If the filename is passed as absolute
(e.g. "kbibtex /tmp/test.bib"), everything is working.

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=689310