This commit-msg hook will copy the ticket number from your branch name and prepend it to your commit msg.

commit_msg.py

#!/usr/bin/python

# 1) copy this file to your computer
# 2) make this file executable:        $ chmod +x /path/to/commit_msg.py
# 3) from the root of your local repo: $ ln -s /path/to/commit_msg.py .git/hooks/commit-msg

import sys
import subprocess
import re

TICKET_PATTERN = '[A-Z]{3}-[\d]+'
POSTAMBLE = 'No commit msg was altered'


try:
    current_ref = subprocess.check_output('git symbolic-ref HEAD', shell=True).decode()
    branch_name = current_ref[len('refs/heads/'):]
except subprocess.CalledProcessError:
    print "Are you in detached HEAD state? Could not determine a branch name. %s" % POSTAMBLE
    sys.exit(0)

match = re.search(TICKET_PATTERN, branch_name)

if match:
    ticket_id = match.group()
    file_name = sys.argv[1]
    with open(file_name, 'r') as f:
        user_supplied_commit_msg = f.read()
    if ticket_id not in user_supplied_commit_msg:
        with open(file_name, 'w') as f:
            f.write('%s %s' % (ticket_id, user_supplied_commit_msg))
        print "JIRA ticket id %s has been prepended to commit msg" % ticket_id
    else:
        print "JIRA ticket id %s found in commit msg. %s" % (ticket_id, POSTAMBLE)
else:
    args = (TICKET_PATTERN, branch_name, POSTAMBLE)
    print 'Could not determine JIRA ticket id using pattern %s and branch name %s. %s' % args