.gitmessage.txt

[__TICKET_NUMBER__]

gistfile1.txt

git config commit.template path/to/.gitmessage.txt

prepare-commit-msg

#!/usr/bin/env ruby
#
# Called by "git commit" with the
# name of the file that has the
# commit message, followed by
# the description of the commit
# message's source.  The hook's
# purpose is to edit the commit
# message file.  If the hook fails
# with a non-zero status,
# the commit is aborted.
#

message_file = ARGV[0]
message = File.read(message_file)

pivotal_task_number = `git rev-parse --abbrev-ref HEAD | grep --only-matching '[0-9]*$'`.strip
replacement_text = pivotal_task_number.length == 0 ? 'NOTICKET' : "##{pivotal_task_number}"

File.open(message_file, 'w') { |f| f.write message.sub('__TICKET_NUMBER__', replacement_text) }