Gets the Trello card ID from your branch name and includes it into the commit message

prepare-commit-msg

#!/bin/bash

# Takes the last token of the branch name (separated with an underscore)
# takes adds is into the commit message.
#
# Note: If you want to about committing from the editor, remove the tag and save the file
#
# 1) Put this file into .git/hooks/prepare-commit-msg
# 2) Make it executable: chmod +x .git/hooks/prepare-commit-message 

CARD_ID=`git rev-parse --abbrev-ref HEAD | sed 's/^.*_//'`
PREV_MSG=`cat "$1"`

if [ "$CARD_ID" != "HEAD" ]; then
  if [[ "$PREV_MSG" != *"[#$CARD_ID]"* ]]; then
    echo "[#$CARD_ID] $PREV_MSG" > $1
  fi  
fi

Idea from here.