gsel ignores edges to nodes

Hello!

I tried to use `gsel` to find "start nodes", i.e. nodes to which have no edges point.
However, `gsel` seems to treat all "Contexts" as if no edges point to the node:

```
import qualified Data.Graph.Inductive as Gr

-- use gsel to select start nodes:
startNodes :: Gr.Gr n e -> [Gr.Context n e]
startNodes =
Gr.gsel $ \x -> case x of
([], _node, _, _outEdges) -> True
_ -> False

-- example graph:
test :: Gr.Gr String ()
test =
Gr.mkGraph
(zip [0..] ["a", "b", "c","d"])
[(0,1,()),(0,2,()),(1,3,()),(2,3,())]
```

now, in ghci:
```
$ > Gr.prettyPrint test
0:"a"->[((),1),((),2)]
1:"b"->[((),3)]
2:"c"->[((),3)]
3:"d"->[]
$ > startNodes test
[([],0,"a",[((),1),((),2)]),([],1,"b",[((),3)]),([],2,"c",[((),3)]),([],3,"d",[])]
```
(gsel selects all nodes)

expected result: `[([],0,"a",[((),1),((),2)])]`
(only node 0 should be selected)

Reproduce the reported behavior in ghci using the provided test graph, startNodes definition, and Gr.prettyPrint output. Inspect the gsel entry point and how it supplies Context values to the predicate; done means gsel distinguishes incoming edges and returns only node 0 for this example.

Written by the indexing model from the issue text.