Test pull request that branch is forked from a branch not merged yet

master   A     B
    o
    |
    |
    o
    | \
    |  \ branch A that will be a pull request
    |   \
    |    o branch A
    |    |
    |    |
    |    o
    |    |
    |    |
    |    o
    |    | \
    |    |   \ branch B that will be another pull request, after branch B is merged
    |    |    \
    |    o     o
    |   /      |
    |  /       |
    | /        |
    o merge pull request of branch A
    |          |
    |          o
    |         /
    |        /
    |       /
    |      /
    |     /
    |    /
    |   /
    |  /
    | /
    |/
    o merge pull request of branch B