Patch anchoring doesn't seem to work with P4 patch

Refer to FSH-3613

Steps to reproduce:

1. Assume that you have added the p4 repository in Fisheye
2. Create a p4 patch via p4 command line

   p4 diff -du > test.txt
   

3. Create a review and upload the patch (e.g see this guide)
4. Clicking the "Anchor to fisheye" link and the error is thrown (see p4patch.png)