Cannot rewrite branch(es) with a dirty working directory

Hi All,

I get a spurious:
"Cannot rewrite branch(es) with a dirty working directory."
trying to filter-branch in a clean git repo (having done a reset). The
error disappears when I do git status.

Log of the shell commands:
bash:jamesb:xl-cbga-20:33083> mkdir org.eclipse.cdt.core.linux.ia64
bash:jamesb:xl-cbga-20:33084> cp -r
../../../CDT_HEAD_GIT/org.eclipse.cdt/.git
org.eclipse.cdt.core.linux.ia64/
bash:jamesb:xl-cbga-20:33085> cd org.eclipse.cdt.core.linux.ia64/
mbash:jamesb:xl-cbga-20:33086> git reset --hard
Checking out files: 100% (11879/11879), done.
HEAD is now at a03d454 Build against a local mirror of the 3.7 p2 repo
bash:jamesb:xl-cbga-20:33087> git filter-branch --subdirectory-filter
core/org.eclipse.cdt.core.linux.ia64 -- master
Cannot rewrite branch(es) with a dirty working directory.
bash:jamesb:xl-cbga-20:33088> git status
# On branch master
nothing to commit (working directory clean)
bash:jamesb:xl-cbga-20:33089> git filter-branch --subdirectory-filter
core/org.eclipse.cdt.core.linux.ia64 -- master
Rewrite d7092b12c93925f6f7c4725a5abc72e55650621c (16/16)
Ref 'refs/heads/master' was rewritten
bash:jamesb:xl-cbga-20:33090> git --version
git version 1.7.3.2

Is there a particular reason why filter-branch thinks the tree is
dirty, and status magically fixes this?

Cheers,
James