title: Running bisect via os.system gives weird error "No testable commit found." while running from bash is ok
body: <p>I'm running bisect like this (tried replacing with <code>subprocess</code>, for good measure. Same results...):</p>
  
  <pre><code>os.chdir(repo_head)
  os.system('git bisect start')
  os.system('git bisect bad  XXX')
  os.system('git bisect good YYY')
  </code></pre>
  
  <p>and getting </p>
  
  <pre><code>No testable commit found.
  Maybe you started with bad path parameters?
  1024
  </code></pre>
  
  <p>on the last command. If I swap the good and bad, I get the error on the last command again. If I ignore, I can run my tests, but nect time I'll enter a good/bad command, I get this error again...</p>
  
  <p>Running directly from bash does not give any errors:</p>
  
  <pre><code>:~/Work (develop)$ git bisect start
  Already on 'develop'
  Your branch is up-to-date with 'origin/develop'.
  :~/Work (develop)$ git bisect good 9b7c546
  :~/Work (develop)$ git bisect bad 63311c8
  Bisecting: 1 revision left to test after this (roughly 1 step)
  [7b7e58f8d13d48e937b84fc7898aa0fadd5fdead] Merged in ZZZ
  </code></pre>
  
  <p>Python 3.5.2, git 2.7.4</p>
  
  <hr>
  
  <p>UPDATE:</p>
  
  <p>Changing my bisect start to bisect start <em>BAD</em> <em>GOOD</em> seems to work fine but I don't get why</p>