Ignore updates to files which are ignored, even if they're indexed.

TL;DR: Git should ignore files if they are in `.gitignore`, regardless of whether or not the file has changed (or been removed).

Use Case

An initial set of "local" config files are committed to a repo. Each developer will pull down this set of files, but changes will be auto-ignored unless the file is force-added after a change is made.

Current Workarounds

1) Use `git update-index --assume-unchanged local-config.xyz` to
ignore local changes to the file.
2) Add templates instead (local-config.template.xyz) and tell each
developer to copy these templates and remove the ".template" portion
of the file name. The file name without the ".template" portion is
added to `.gitignore`.

The current workarounds rely on the developer to know/remember what to do. It would be nice if we could do this instead:

1) Create the initial `config-local.xyz`.
2) Add the new file to the index (`git add`).
3) Add the same path to `.gitignore`, and add the changes to the index.
4) Commit the staged changes.
5) Edit the `config-local.xyz` file.
6) Run `git status` -- git will not suggest that the file was changed
since it is in the `.gitignore`. If 7) The user specifically made
changes they wish to commit, the can force-add the ignored file.

Doing a search for [git commit file only once then ignore changes] will show that other people wish for the same feature.

Thanks!

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