commit -amend fails on first commit

[zombiezen]

Steps to Reproduce

$ gg init
$ touch foo.txt
$ gg add foo.txt
$ gg commit -m first
$ echo hello > foo.txt
$ gg commit -amend

Expected Behavior

Editor is opened for amending

Actual Behavior

gg crashes with:

gg: diff status:
fatal: ambiguous argument 'HEAD~': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]'

Version Info

gg version 0.7.1, built on 2019-02-13T18:53:28Z
go: go1.11.5 gc linux/amd64
git version 2.20.1