how can I customize git's merge commit message?

Question

Every time I do a merge I need for a merge commit to be generated and I would like it to have more than just the summary of all the commits.

My question is how can I format git-fmt-merge-msg or what determines this automated message (I can do this manually after a commit by amending it and using git-log --pretty=format:'...')

For example I would like to format it as such:

 Merge branch 'test'  
    * test:  
      [BZ: #123] fifth commit subject  
      [BZ: #123] fourth commit subject  
      [BZ: #123] third commit subject  
      [BZ: #123] second commit subject  
      [BZ: #123] first commit subject  

 __________________________________________
 Merge details:  
     [BZ: #123] fifth commit subject  
               at 2010-06-30 11:29:00 +0100  
       - fifth commit body  

     [BZ: #123] fourth commit subject  
               at 2010-06-30 11:22:17 +0100  
       - fourth commit body  

     [BZ: #123] third commit subject  
               at 2010-06-30 11:21:43 +0100  
       - third commit body  

     [BZ: #123] second commit subject  
               at 2010-06-30 11:21:30 +0100  
       - second commit body  

     [BZ: #123] first commit subject  
               at 2010-06-30 11:29:57 +0100  
       - first commit body

Answer 1

I'm aware this isn't answering the original question, but for the benefit of git noobs like myself who reach this page because it's currently the first Google result for "git change merge commit message", I'll mention that it is possible to:

git commit --amend -m"New commit message"

to change the commit message of a merge commit without losing the link to any of the parents of the merge commit.