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

edit:

for non-ideal fix see answer below

but the real question is how to modify the information generated by the 'fmt-merge-msg'?

Answer 1

You could try defining a prepare-commit-msg hook (the sample one does generate some custom "default commit messages")

Answer 2

There's also a --log option for git-merge now, which does half of what you want - it places the shortlog (commit summaries) in the merge message. A full solution will have to use a hook like in VonC's answer, though.

Answer 3

I've found there are two ways for solving this problem

note: don't use both at the same time, as if the commit fails to merge it will add the log again to the bottom.

personal note: I'm using the first solution as it relies entirely on git's hooks and config properties, instead of an external script.
For a real solution one would have to extend a git command named 'fmt-merge-msg' that generates the oneline descriptions when passing the --log option (if you really need this solution you'll have to create your own patch (for git) and compile it from source).

1. using prepare-commit-message as VonC suggested
this solution has the problem that you need to interrupt the commit and then commit manually

setting an alias that will build the desired commit message:

[alias]  
lm = log --pretty=format:'%s%n   by %C(yellow)%an%Creset (%ad)%n %n%b' --date=local

creating the prepare-commit-msg hook by creating an executable prepare-commit-msg in $GIT_DIR/hooks/ (example script below)

#!/bin/sh
#...

case "$2,$3" in  
  merge,)  
  echo "Merge details:" >> $1  
  echo "" >> $1  
  git lm ORIG_HEAD..MERGE_HEAD >> "$1" ;;  
*) ;;  
esac  

one should define an alias commit msg such as

[alias]  
m = merge --no-ff --no-commit

2. using a custom command that will generate the merge automatically
(using the lm alias created in 1.)

#!/bin/sh

echo ""
echo "merge with commit details -- HEAD..$1"
git merge --no-ff --no-log -m "`git lm HEAD..$1`" --no-commit $1

and then execute a rather rigid command:

./cmd-name <branch to merge>

if you still wish to have the oneline description of the commits you'll need to add new commands or whatever to the -m argument (if you use --log then it will be generated on the bottom)