[BUG] b9c8e7f2fb6e breaks git bisect visualize

Commit b9c8e7f2fb6e ("prefix_ref_iterator: don't trim too much") breaks git bisect visualize, this reproduces the bug:

  $ git bisect start
  $ git bisect bad
  $ git bisect good HEAD^^
  $ git bisect visualize
  fatal: BUG: attempt to trim too many characters
  $

Reverting b9c8e7f2fb6e makes git bisect visualize work again.

Tested on Debian GNU/Linux 8.8 (jessie).

Øyvind

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