ember-cli-update: Doesn't work when ember project is subfolder

When I run ember update I get this output:

Cloning into bare repository '/var/folders/3s/kt39dyns0ngc361qvkxznlfw0000gn/T/tmp-138173bpZj5aJOoJI/.git'...
Switched to a new branch 'd3bc36e0-939f-11e7-bc82-799873862e5d'
Note: checking out 'ac5673b56583dfee01ceae3783467e5ca4dac25c'.

You are in 'detached HEAD' state. You can look around, make experimental
changes and commit them, and you can discard any commits you make in this
state without impacting any branches by performing another checkout.

If you want to create a new branch to retain commits you create, you may
do so (now or later) by using -b with the checkout command again. Example:

  git checkout -b <new-branch-name>

HEAD is now at ac5673b... 2.7.0
File: /bin/sh
Command failed: git commit -m "diff"

Error: Command failed: git commit -m "diff"

    at checkExecSyncError (child_process.js:472:13)
    at Object.execSync (child_process.js:512:13)
    at run (/<path-to-app>/node_modules/ember-cli-update/node_modules/git-diff-apply/src/run.js:8:19)
    at ncp.then.err (/<path-to-app>/node_modules/ember-cli-update/node_modules/git-diff-apply/src/index.js:62:5)
    at process._tickCallback (internal/process/next_tick.js:109:7)

Oddly it can’t commit this for some reason, but there is another commit preceding the error.

I can only think of two possible reasons for the failed update at this moment:

1. Our project is at Ember 2.7 right now. Perhaps it’s too old for the tool to work with?
2. The app is inside a directory in a bigger git repository, which might prevent the underlying tools to do their git magic?

It also fails if I run the global ember-cli-update with the same error and slightly bigger output containing information about the system.

Please let me know if this is a valid issue and if I can assist further.