[merge] trivially add KnitVersionedFiles repr

[Vincent Ladeuil]

>>>>> "martin" == Martin Pool <mbp at canonical.com> writes:

    martin> This class does sometimes occur in errors like
    martin> bzr: ERROR: Revision
    martin> {('mbp at sourcefrog.net-20080502082540-jf964t0zy107xut2',)} not present
    martin> in "<bzrlib.knit.KnitVersionedFiles object at 0x8d0394c>".

    martin> it would be useful to know just where the problem is.

BB:approve