fast-import-filter doesn't work well with streams from branches

[Alex Usov]

I was trying to use fast-import-filter to filter out a few files from the repo.
However it seems that it doesn't play nice with partial streams (e.g. streams containing just the specific branch).

Example:
we export mainline first:
> bzr fast-export --marks=main.marks main main.fi.gz

then we export just the side-branch:
> bzr fast-export --import-marks=main.marks --export-marks=branch.marks branch branch.fi.gz

Now if I will run branch.fi.gz through fast-import-filter on it's own, it will strip "from :123" qualifier from the very first entry in the stream.