"git log --first-parent" shows parents that are not first

The result given by "git log --first-parent" ('next' version) is
unexpected to me.

  % git rev-parse origin/next
  4eddac518225621c3e4f7285beb879d2b4bad38a
  % git log --abbrev-commit --pretty=oneline --first-parent origin/next^..origin/next
  4eddac5... Merge branch 'master' into next
  1f8115b... Merge branch 'maint'
  ca1c991... Merge branch 'sg/merge-options' (early part)
  31a3c6b... Merge branch 'db/learn-HEAD'
  a064ac1... Merge branch 'jn/webfeed'
  d576c45... Merge branch 'cc/help'
  ca1a5ee... Merge branch 'dm/cherry-pick-s'
  4c4d3ac... Merge branch 'lt/dirmatch-optim'
  c5445fe... compat-util: avoid macro redefinition warning
  eb120e6... compat/fopen.c: avoid clobbering the system defined fopen macro
  bac59f1... Documentation: bisect: add a few "git bisect run" examples
  d84ae0d... Documentation/config.txt: Add git-gui options
  921177f... Documentation: improve "add", "pull" and "format-patch" examples
  c904bf3... Be more careful with objects directory permissions on clone

I asked for the log between one commit before the tip of "origin/next" and the tip of the branch, following only the first-parent links.  v1.5.5 is not broken and shows the expected result:

  % ~/git-v1.5.5/bin/git log --abbrev-commit --pretty=oneline --first-parent origin/next^..origin/next
  4eddac5... Merge branch 'master' into next

-- 
Nanako Shiraishi
http://ivory.ap.teacup.com/nanako3/

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