rebase -continue adds untracked files

[zombiezen]

Steps to Reproduce

$ gg init
$ touch foo.txt
$ gg add foo.txt
$ gg commit -m "first"
$ touch bar.txt
$ gg add bar.txt
$ gg commit -m "second"
$ gg histedit HEAD~
# edit HEAD commit
$ touch baz.txt
$ gg histedit -continue
# don't change commit message

Expected Behavior

Equivalent commit at HEAD.

Actual Behavior

baz.txt is committed.

Version Info

gg version 0.7.1, built on 2019-02-13T18:53:28Z
go: go1.11.5 gc linux/amd64
git version 2.11.0