How to "pre-populate" a git message

Once in a while, I would like to run some scripts that will
automatically generate some files that I keep maintained in my
repository.  (Yes, I know that I shouldn't have to keep them version
controlled if I can regenerate them, but it's easier to check out old
versions of the files than it is to check out old versions of the
repository, wait 1/2 hour to regenerate the files, and then look at
them).

I also like to review the files before they get committed.

Is there any way I can "pre-populate" the commit message so that I can
run my "regenerate" command, review the differences, and then commit
the changes with my prewritten message?

--wpd