git reup dies poorly with uncommitted changes

[aarongable]

Steps to reproduce
git nb foo
echo 'a change' >> some_file
[wait for changes on origin/master]
git reup

It dies in a fire:

Traceback (most recent call last):
File "/usr/local/google/home/agable/dotfiles/git/scripts/reup.py", line 108, in
sys.exit(main())
File "/usr/local/google/home/agable/dotfiles/git/scripts/reup.py", line 92, in main
clean_branch(branch, parent, starting_refs[branch])
File "/usr/local/google/home/agable/dotfiles/git/scripts/reup.py", line 32, in clean_branch
if rebase(parent, starting_ref, branch, abort=True).success:
File "/usr/local/google/home/agable/dotfiles/git/scripts/reup.py", line 22, in rebase
run_git('rebase', '--abort')
File "/usr/local/google/home/agable/dotfiles/git/scripts/common.py", line 180, in run_git
ret = check_output(cmd, **kwargs)
File "/usr/local/google/home/agable/dotfiles/git/scripts/common.py", line 170, in check_output
raise CalledProcessError(retcode, cmd, output=output, out_err=out_err)
common.CalledProcessError: Command "('git', 'rebase', '--abort')" returned non-zero exit status 1