VcView: use extension with temp files so syntax highlight correct (#459)

When comparing a file with its previous version, sometimes the syntax highlighting is incorrect in the previous version. This is because the algorithm used by the "LanguageManager.get_language_from_file" (and ultimately gio) uses the file extension first to determine the file language. When the local file has an extension and the remote file does not, the language manager might pick a different language for the remote.

Since the files being compared are the same one, just
different versions, use the local extension for the
remote temp file.

See Issue #459 (closed)