Add binding for git_merbe_bases_many()

[otac0n]

I suggest adding a new method, FindMergeBases(), since it changes the return type to IEnumerable.

See: http://stackoverflow.com/questions/31320882/finding-ancestry-path-of-a-merge-in-libgit2sharp

[carlosmn]

Note that git_merge_bases_many() is not the same as --octopus. An octopus-merge-base is an iterative best-merge-base, which is is what git_merge_base_octopus() does.