You probably want to set up a prepare-commit-msg hook on your local repository. It might look like this (say the branches are named ‘work-on-ticket-XXXX’:
#!/bin/sh
ORIG_MSG_FILE="$1"
TEMP=`mktemp /tmp/git-XXXXX`
TICKETNO=`git branch | grep '^\*' | cut -b3-`
(echo "Work on ticket #$TICKETNO"; cat "$ORIG_MSG_FILE") > "$TEMP"
cat "$TEMP" > "$ORIG_MSG_FILE"
Put something like that (marked executable) in .git/hooks/prepare-commit-msg. You may have to adjust and elaborate on it of course.