Calculate "N commits behind" using `--first-parent` flag

Requirements: Empty repository with a remote on machine M1 with git-flow initialised (`develop` branch is required).

Steps to reproduce:

1. M1: create a feature branch;
2. M1: check out the feature branch;
3. M1: make few commits in the feature branch (N > 1);
4. M1: push the feature branch to remote;
5. M2: pull the changes and checkout the develop branch;
6. M2: merge the feature branch into develop;
7. M2: push the merge commit to origin;
8. M1: fetch (not pull) the changes from origin;

Now observe, that develop branch on M1 is N commits behind the remote one when it actually should be only 1 commit behind (the 1 commit that is merge between feature and develop branches). The SourceTree counts the commits using the last, not the first parent of the merge commit. To fix this you can use `--first-parent` flag that appeared in git 1.8.4.

Please ignore this improvement request if that's intended behavior.