qlog file1 file2: unable to get diff of revision if some file doesn't exist yet

[Alexander Belchenko]

In my rng-talker/trunk @ work branch doing `bzr qlog spi.c spi.h` I have what I have on screnshot. The first shown revision is revno.5 and it has spi.c added, but not spi.h. Clicking on Diff button or double click on such revision produces traceback:

bzr: ERROR: The file id "spi.h-20090428062949-9w732zqt2850nti3-1" is not present in the tree None.

The only workaround is double-click on the file in the changed files report area (bottom right of qlog).

I think the right thing here is to explicitly check whether a requested file presents in the revision tree for selected revision. But things will faster going tricky if there is multiple revisions checked. Maybe we need some simpler check?