Johannes Schindelin [off-list ref] writes:

Of course, there is a fourth of "two other" possibilities:

Make a script calling git-commit with "-F - -e" and pipe your generated 
template into it.

Use this script whenever you want to create a new commit.

I think that the approach has one huge advantage.  Commands
other than "git commit" itself ("git merge", "git rebase", "git
am", etc.) do call "git commit" to record the changes they made.
I suspect these command would not want this template behaviour,
and not adding this custom commit message "feature" to "git
commit" would avoid the risk of breaking them.

At the same time, this exact issue could be a drawback.  Some of
them _might_ want it.  But in that case, the the custom template
"hook" needs to be told _why_ it is being called, so that it can
adjust its behaviour.