echo JIRA >~/txt/git-commit-template.txt
And one of these commands
git config commit.template ~/txt/git-commit-template.txt
git config --global commit.template ~/txt/git-commit-template.txt
or put in in .git/config (per repo) or $HOME/.gitconfig (global):
[commit]
template = /Users/mh/txt/git-commit-template.txt
blogodex = {"toc" : "git", "idx" : "commit template"};