Type check the tree value for Rugged::Index#read_tree

[adelcambre]

libgit2 does no type checking in git_index_read_tree which means that
index.read_tree will segfault if you pass something other than a tree.

This ensures that the value passed is actually a tree.

[adelcambre]

Also, I'm not sure if this should be handled in libgit2 as well. I was chatting with @arrbee and he indicated that rugged is a better place for this sort of type checking.

[carlosmn]

libgit2, being in C, has static type checking. The function accepts a git_tree * so any decent C code should have the right type anyway. Of course, being in C, you could pass in a pointer to whatever and still make it crash. Since it's rugged the one that's casting between types, I agree it's the one which should be making sure the passed-in object is the right one.

[arthurschreiber]

Looks great, thanks!