I tried running it here and I got:

<code>
$ mocha test/spec.git-api.diff.js
connect.multipart() will be removed in connect 3.0
visit https://github.com/senchalabs/connect/wiki/Connect-3.0 for alternatives
connect.limit() will be removed in connect 3.0

  ....1
........

  × 1 of 12 tests failed:

  1) git-api diff diff on image file should work:
     Error: expected "Content-Type" matching /json/, got "image/png"
      at Test.assert (d:\github\ungit\node_modules\supertest\lib\test.js:189:25)
      at d:\github\ungit\node_modules\supertest\lib\test.js:120:10
      at Test.Request.callback (d:\github\ungit\node_modules\supertest\node_modules\superagent\lib\node\index.js:603:30)
      at Test.<anonymous> (d:\github\ungit\node_modules\supertest\node_modules\superagent\lib\node\index.js:134:10)
      at Test.EventEmitter.emit (events.js:95:17)
      at IncomingMessage.<anonymous> (d:\github\ungit\node_modules\supertest\node_modules\superagent\lib\node\index.js:738:12)
      at IncomingMessage.EventEmitter.emit (events.js:117:20)
      at _stream_readable.js:920:16
      at process._tickCallback (node.js:415:13)
</code>

So at least on my machine it seems like it's because common.get is looking for a response with json content type when in fact you want image/png.