R1:182a14628088

Set "history.immutable" to "false" explicitly in .arcconfig

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…