The corresponding test is attached.
It creates a repo, creates a branch, does some commits either to the created branch and to master, then merges master to the created branch. After merge it tries to calculate the number of commits between the branch and master. Because master has been already merged -- the branch is not behind the master, i.e. corresponding commit count should be 0. But 'Iterables.size' after a 'log' call returns the number of commits in the master minus 1.
Moreover, if 'sleep' in the line 43 is uncommented -- the bug will go away, and 'behind' is equal to 0.