* Add `builtin_immutable_heads()` in the `revsets.toml`.
* Redefine `immutable_heads()` in terms of `builtin_immutable_heads()`
* Warn if user redefines `builtin_immutable_heads()`, `mutable()` or
`immutable()`.
* Update module constant in revset_util.rs from BUILTIN_IMMUTABLE_HEADS
to USER_IMMUTABLE_HEADS to avoid confusion since it points at
`immutable_heads()` **and** we now have a revset-alias
literally named `builtin_immutable_heads()`.
* Add unittest
* Update CHANGELOG
* Update documentation.
Fixes: #4162
a6d8009097