Restore the ability to disable incremental compilation

It became the default in debug mode in the last Rust/Cargo update.

python/servo/command_base.py

@@ -278,7 +278,7 @@ def resolverelative(category, key):
         self.config["build"].setdefault("debug-mozjs", False)
         self.config["build"].setdefault("ccache", "")
         self.config["build"].setdefault("rustflags", "")
-        self.config["build"].setdefault("incremental", False)
+        self.config["build"].setdefault("incremental", None)
         self.config["build"].setdefault("thinlto", False)
 
         self.config.setdefault("android", {})
@@ -439,6 +439,8 @@ def package_dir(package):
 
         if self.config["build"]["incremental"]:
             env["CARGO_INCREMENTAL"] = "1"
+        elif self.config["build"]["incremental"] is not None:
+            env["CARGO_INCREMENTAL"] = "0"
 
         if extra_lib:
             if sys.platform == "darwin":

servobuild.example

@@ -40,8 +40,10 @@ debug-mozjs = false
 # Any optional flags that will be added to $RUSTFLAGS
 #rustflags = ""
 
-# Enable rustc’s incremental compilation
+# Enable or disable rustc’s incremental compilation
+# The default is enabled in debug mode only.
 #incremental = false
+#incremental = true
 
 # Whether to use ThinLTO or not
 #thinlto = false