Summary:
If contributors use "history.immutable" by default (for example, in user/global config) and then try to contribute to Phabricator, they might land in a way that creates merge commits, which we'd prefer to avoid.

To make this a bit harder to do unintentionally, set "history.immutable" to "false" explicitly in `.arcconfig`. This will override any user or system setting and keep `land` in squash mode in this project.

This might also be a problem in general, and maybe in the long term we need something like one of these warnings:

> (when arc landing) You haven't landed changes to this project before, are you sure you want to create a merge commit / are you sure you want to squash?

...or:

> (when doing anything) This project doesn't have a history.immutable setting; you should probably set one so user settings don't win out.

...or just not let this be set at the user/system level, although that's annoying in some cases. Also maybe this:...