Graphs.merge doesn't actually merge graphs!

[aphyr]

Graphs.merge creates a result variable, but then mutates variable a instead of result, and returns result. Merge winds up being a noop for immutable graphs. :-O

bifurcan/src/io/lacuna/bifurcan/Graphs.java

Line 191 in 32b3fbf

a = a.link(src, dst, b.edge(src, dst), merge);