A bash script to pre-populate a tag annotation in vim with the first line of all commits since last tag:

tag.sh

#!/bin/bash
# Tag the current commit with an annotation generated from the first line of the commit messages of all
# commits since the last tag. Default tag annotation is loaded into vim buffer and can be edited.
# Once you are happy with the message write the buffer and quit vim (type ":wq") and the tag will be generated
# 
# Usage: 
#
# tag 1.2.3
COMMITS_SINCE_LAST_TAG=$(git log `git describe --tags --abbrev=0`..HEAD --oneline)
echo "# Version $1"$'\n\n'Changelog:$'\n\n'"$COMMITS_SINCE_LAST_TAG" | vim - +'file TAG_ANNOTATION'
if [ -f TAG_ANNOTATION ]; then
    git tag $1 --file=TAG_ANNOTATION
    rm TAG_ANNOTATION
    echo "Commit successfully tagged $1 with given annotation"
else
    echo "Aborted. Tag not added."
fi