base option doesn't always work for branch creation

[LMG]

Sometimes, the base option fails to find a common commit.
The result is that the some commits from master are missing from the pushed branch, rendering all the shas different.

I believe this was introduced here: 6cd2bdb#diff-11986cd01433c2eaf2865ad8a15e712deaf6bdf38800f7fd8409f89b21c39dca

As you can see, the "initial" commit is missing from the pushed branch in unrelated_leak.t

[LMG]

should be fixed by #555