trunk update required before merging branch changes

I'm trying to write a shell script to demonstrate the merge capabilities
of Subversion 1.5. For some reason, before I commit the merge, I have to
do an update. Can anyone explain why what seems to me like a "special"
or "extra" update is required before the merge?

Here's an outline of what I'm scripting:

1. set up repository on local filesystem
2. make a change on trunk
3. branch from trunk
4. make a change on the branch
5. make a change on the trunk
6. merge branch changes to trunk

I must update the trunk before step #6 or I get the following error:

  svn: Commit failed (details follow):
  svn: Directory '/trunk' is out of date

The complete shell script is here: http://pastey.net/105987

If I pause right before trying to commit the merge and inspect the trunk
dir in the working copy, I see:

  Revision: 3
  Last Changed Rev: 2

And if I actually do the update:

  Revision: 5
  Last Changed Rev: 5

I'm using:

  Operating System: Ubuntu 8.10
  Software: svn, version 1.5.1 (r32289) [command line client]

-- 
Adam Monsen
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