Add merge-base fixtures

[dpordomingo]

blocks src-d/go-git#1097

The following tests consider this history having two root commits: V and W

V---o---M----AB----A---CD1--R---C--------S-------------------Q < master
               \         \ /            /                   /
                \         X            GQ1---G < feature   /
                 \       / \          /     /             /
W---o---N----o----B---CD2---o---D----o----GQ2------------o < dev

MergeBase
----------------------------
passed  merge-base
 M, N               Commits with unrelated history, have no merge-base
 A, B    AB         Regular merge-base between two commits
 A, A    A          The merge-commit between equal commits, is the same
 Q, N    N          The merge-commit between a commit an its ancestor, is the ancestor
 C, D    CD1, CD2   Cross merges causes more than one merge-base
 G, Q    GQ1, GQ2   Feature branches including merges, causes more than one merge-base

Independents
----------------------------
candidates        result
 A                 A           Only one commit returns it
 A, A, A           A           Repeated commits are ignored
 A, A, M, M, N     A, N        M is reachable from A, no it is not independent
 CD1, CD2, M, N    CD1, CD2    M and N are reachable from CD2, so they're not
 C, G, dev, M, N   C, G, dev   M and N are reachable from G, so they're not
 A, A^, A, N, N^   A, N        A^ and N^ are rechable from A and N


IsAncestor
----------------------------
passed   result
 A^^, A   true      Will be true if first is ancestor of the second
 M, G     true      True because it will also reach G from M crossing merge commits
 A, A     true      True if first and second are the same
 M, N     false     Commits with unrelated history, will return false