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 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:

> (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.

Test Plan:
  - Ran `arc set-config history.immutable true` to set the flag in my user settings.
  - Ran `arc get-config` to verify that the setting overrode system/user settings:

```
epriestley@orbital ~/dev/phabricator $ arc get-config history.immutable --verbose
history.immutable

    If true, arc will never change repository history (e.g., through amending
    or rebasing). Defaults to true in Mercurial and false in Git. This
    setting has no effect in Subversion.

       Example Value: false

       Current Value: false
      Current Source: project

         local Value: -
       project Value: false
          user Value: true
        system Value: -
       default Value: -
```

Reviewers: chad, fooishbar

Reviewed By: fooishbar

Differential Revision: https://secure.phabricator.com/D16364

.arcconfig

@@ -1,4 +1,5 @@
 {
   "phabricator.uri": "https://secure.phabricator.com/",
-  "load": ["src/"]
+  "load": ["src/"],
+  "history.immutable": false
 }