Provide the smallest possible cycle in deadlock detector reports

Originally reported on Google Code with ID 51

```
When a lock-order inversion occurs (usually detected through DFS) we should provide
the smallest possible cycle (to ease the burden of debugging it).

It's possible to have a lock orders established: A->B->C, A->C. If this order is then
broken by locking C->A, then a DFS could provide C->A->B->C, even though C->A->C would
be sufficient and easier to parse/debug.

```

Reported by `pbos@google.com` on 2014-03-05 16:04:34

Start by tracing the deadlock detector's DFS-based report generation; the issue does not name a source file or test. Use the A→B→C, A→C, then C→A example to determine how the reported cycle is selected. Done means lock-order inversion reports show the smallest sufficient cycle.

Written by the indexing model from the issue text.