Add merge base command

[dpordomingo]

Uses the code from #1097 to implement a command, mimicking git merge-base.

Help with a full description of the implemented functionalities is shown running merge_base --help.

usage:

$ ./merge_base

Returns the merge-base between two commits:

usage: merge_base <path> <commitRev> <commitRev>
   or: merge_base <path> --independent <commitRev>...
   or: merge_base <path> --is-ancestor <commitRev> <commitRev>
   or: merge_base --help

    (no option)       lists the best common ancestors of the two passed commits
    --independent     list commits not reachable from the others
    --is-ancestor     is the first one ancestor of the other?
    --help            show the full help message of merge_base

[dpordomingo]

Travis is failing https://travis-ci.org/src-d/go-git/jobs/511121578#L668 because the new test cases depend on new fixtures, to be merged by src-d/go-git-fixtures#13

[dpordomingo]

@mcuadros the _example command is again ready after #1097 was merged.
It also includes the "integration tests" for the merge_base feature.

[yelirekim]

👋

Is this PR ready for merge?

It would be really great if we could get the ability to find merge bases put out in a release at some point. We have several applications that would benefit from it downstream.

[yelirekim]

🎉

[zhaopengme]

🎉