title: Cannot combine commits using TortoiseGit
body: <p>I have two branches with several commits each. On one branch, I can go to the log, select two commits, and TortoiseGit shows "combine to one commit" in the context menu. On the other branch this option does not show in the context menu. Both sequence of commits is very similar; add file then modify it, so there is no difference really between the branches.</p>
  
  <p>What factors would cause this "combine to one commit" to not be available? I'm wondering if I should just switch to the command line.</p>