run gvim in foreground mode

--diff-prog gvim will show a broken file, because as gvim forks out, the
temporary file got removed on spec-cleaner exit. This forces a
foreground mode for everything started on gvim ...

References: https://github.com/openSUSE/spec-cleaner/issues/7
Merge pull request #10 from mvyskocil/master

run gvim in foreground mode