How can I make git ignore future revisions to a file?

What you are searching for is git update-index --assume-unchanged default_values.txt.

See the docs for more details: http://www.kernel.org/pub/software/scm/git/docs/git-update-index.html

The approach I've generally seen is to create a file with a different name, eg: default_values_template.txt and put default_values.txt in your .gitignore. Instruct people to copy default_values_template.txt to default_values.txt in their local workspaces and make changes as needed.

As many others have mentioned, a good modern solution is:

git update-index --skip-worktree default_values.txt

That will ignore changes to that file, both local and upstream, until you decide to allow them again with:

git update-index --no-skip-worktree default_values.txt

You can get a list of files that are marked skipped with:

git ls-files -v . | grep ^S

Note that unlike --skip-worktree, the --assume-unchanged status will get lost once an upstream change is pulled.