How to make "git merge --log" stop flattening merge commits

Question

I am using a successful git branching model, where you have master branch only consisting from merge commits with version tags, and all the development goes on develop branch.

The develop branch also consists of merge commits (feature branches) and ordinary commits (small individual changes).

When I perform a new version release, I want git to make me a chengelog. git merge develop --log does this, but this "unrolls" merge commits on develop, resulting in a flat history.

I want to make 'git merge --log' use only commits on develop, is this possible?

Examples:

What I atually get now when merging:

Merge branch 'develop' into master

* develop:
  fix 1 on branch develop
  fix 2 on branch develop
  fix 1 on branch b1
  fix 2 on branch b1
  fix 3 on branch b1
  fix 1 on branch b2
  fix 2 on branch b2
  fix 1 on branch b3

What I want it to be:

Merge branch 'develop' into master

* develop:
  fix 1 on branch develop
  fix 2 on branch develop
  Merge branch 'b1' into develop
  Merge branch 'b2' into develop
  Merge branch 'b3' into develop

Answer 1

To get the log you want for an inflight merge, use

git log --pretty='  %s' --first-parent ..MERGE_HEAD  # with e.g. `-10` to limit to 10

or to see what that'd get from an existing merge use merge^1..merge^2 as your range.

The quickest way to get to a smoketest is probably git merge --log=1 to generate the right boilerplate, then in vim it'd be

:/^\*/+,+!git log --pretty='  \%s' --first-parent ..MERGE_HEAD

to fill in the log the way you want. To automate it you'll need to use the prepare-commit-msg hook, something along the lines of

case $2 in
merge)
        git log --pretty='  %s' --first-parent ..MERGE_HEAD >${tempfile=`mktemp`}
        sed -i '/^\*/,/^$/ { //!d }; /^\*/ r '$tempfile  "$1"
        rm $tempfile
        ;;
esac

edit: on GNU/anything, the hook gets simpler because the sed's buffier:

case $2 in
merge)
        git log --pretty='  %s' --first-parent ..MERGE_HEAD \
        | sed -i '/^\*/,/^$/ { //!d }; /^\*/ r /dev/stdin' "$1"
        ;;
esac