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?
(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:
(when arc landing) You're about to create a merge commit, but the last Differential change in history squashed instead. Merge anyway?
However, we haven't seen this as a widespread issue outside of this project yet (and even in this project I think it has only happened 2-3 times), so just put up a guard rail in our own configuration for now.