[BUG] mincut gives incorrect results for some SimpleGraphs

[hansenjakob]

Description of bug The mincut function produces incorrect partitions for some graphs.

How to reproduce The following code produces incorrect results:

edge_list = [(1,2), (1,3), (1,4), (1,5), (2,6), (3,4), (3,6), (4,6), (5,6)]
g = SimpleGraphFromIterator([Edge(e...) for e in edge_list])
labels, cut_size = mincut(g)

Expected behavior cut_size should be at most 2 because both vertex 2 and vertex 5 have degree 2.

Actual behavior cut_size is 3. In this instance, the cut given separates vertex 4 from the rest of the graph.

In experiments with random graphs, this behavior arises fairly frequently.

Version information

• output from versioninfo() surrounded by backticks (``)

  Julia Version 1.6.0
  Commit f9720dc2eb (2021-03-24 12:55 UTC)
  Platform Info:
  OS: macOS (x86_64-apple-darwin19.6.0)
  CPU: Apple M1
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-11.0.1 (ORCJIT, westmere)
  Environment:
  JULIA_EDITOR = code
  JULIA_NUM_THREADS = 

• output from ] status LightGraphs surrounded by backticks (``)

      Status `~/.julia/environments/v1.6/Project.toml`
  [093fc24a] LightGraphs v1.3.5