Get rid of unnecessary `begin`/`end` in PairMatrix#pairs

Change-Id: Ie00aa8327cc97a837c9d89e23c8f682f857098dc
Reviewed-on: https://gerrit.causes.com/26076
Reviewed-by: Greg Hurrell <greg@causes.com>
Tested-by: Greg Hurrell <greg@causes.com>
Fix bug with double-counting solo commits in PairMatrix

I mistakenly thought that an array like `['Joe', 'Joe']` would get
permuted to `[['Joe', 'Joe']]`, but it actually gets permuted to
`[['Joe', 'Joe'], ['Joe', 'Joe']]`.

Will follow-up with a spec.

Change-Id: Ia897cd89d4d969283fbf54abb379cb5e32bbf485
Reviewed-on: https://gerrit.causes.com/26077
Reviewed-by: Greg Hurrell <greg@causes.com>
Tested-by: Greg Hurrell <greg@causes.com>