What's the URL of the page containing the problem?
http://gsoc.review-board.org/r/new/
Using Mercurial, you can define in your .hgrc:
[diff]
git = 1
The git's diff format is not supported by Review Board. It complains
it doesn't seem a diff file.
The next is a diff between two diff from the same repository. One is using
the git format and the another is using the default for Mercurial.
$ diff -c fix-actions-in-non-repo.*
*** fix-actions-in-non-repo.diff 2008-05-27 00:37:53.000000000 -0400
--- fix-actions-in-non-repo.patch 2008-05-27 00:42:05.000000000 -0400
***************
*** 1,6 ****
! diff -r 4d6880646a66 contrib/nautilus-thg.py
! --- a/contrib/nautilus-thg.py Sun May 04 23:20:27 2008 -0500
! +++ b/contrib/nautilus-thg.py Tue May 27 00:37:53 2008 -0400
@@ -168,8 +168,13 @@
repo = self.get_repo_for_path(path)
cwd = os.path.isdir(path) and path or os.path.dirname(path)
--- 1,6 ----
! diff --git a/contrib/nautilus-thg.py b/contrib/nautilus-thg.py
! --- a/contrib/nautilus-thg.py
! +++ b/contrib/nautilus-thg.py
@@ -168,8 +168,13 @@
repo = self.get_repo_for_path(path)
cwd = os.path.isdir(path) and path or os.path.dirname(path)