Writing TTrees is getting the branch type from the pattern of values, not the Awkward type

[jpivarski]

4.1.2

>>> import uproot
>>> import awkward as ak
>>> f = uproot.recreate("whatever.root")
>>> a = ak.Array([[1, 2, 3], [4, 5, 6]])
>>> f["tree"] = {"branch": a}
>>> f["tree"].show()
name                 | typename                 | interpretation                
---------------------+--------------------------+-------------------------------
branch               | int64_t[3]               | AsDtype("('>i8', (3,))")

That should be a branch of type int64_t[] with a counter nbranch because the array type is

2 * var * int64

not

2 * 3 * int64

It doesn't matter that both lists happen to have the same length. (In a subsequent batch, they might not.)