Add branch name to commit message.

commit-msg

#!/bin/sh
# prepend branch name to message
# aborts on empty message
# not applied for branch `main`
# by-passe by prefixing with '!!'
# (in which these are removed)
two=`head -c 2 "$1"`
case ${two:-#} in
  \#*) : >"$1"; exit 1;;
  \!!) sed -i 1s/^..// "$1"; exit 0;;
esac
branch=`git rev-parse --abbrev-ref HEAD`
[ main = $branch ] || sed -i 1s/^/$branch:\ / "$1"