filter-branch: do not consider diverging submodules a 'dirty worktree'

At the end of filter-branch in a non-bare repository, the work tree is updated with "read-tree -m -u HEAD", to carry the change forward in case the current branch was rewritten. In order to avoid losing any local change during this step, filter-branch refuses to work when there are local changes in the work tree. This "read-tree -m -u HEAD" operation does not affect what commit is checked out in a submodule (iow, it does not touch .git/HEAD in a submodule checkout), and checking if there is any local change to the submodule is not useful. Staged submodules _are_ considered to be 'dirty', however, as the "read-tree -m -u HEAD" could result in loss of staged information otherwise. Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>