#27 delay deleting temporary files for external diff tool

[Rob]

If you set the external diff program to 'gvimdiff' or 'terminal -e vimdiff', you see the diff of two empty files. This is because these run asynchronously and qgit deletes the files before they can be opened. A workaround is to write a wrapper script with a 'sleep 1' after the diff command. Putting the delay in qgit would be simpler for users and avoid confusion.