DiffMatcher throws exception if a matching file does not exist on either buggy or working project

[bchenghi]

In tregression.separatesnapshots.diff.DiffParser#parseDiff, it sets the the file path to the missing file as "dev/null". tregression.separatesnapshots.diff.FilePairWithDiff#getDeclaringCompilationUnit is called when clicking on a TraceNode in Trace View, which will throw an exception, as it tries to find a file at path "dev/null".