Phabricator: enable "history.immutable":

With certain arc workflows leaving history.immutable as false
	results in using the upstream template instead of our usual
	commit template.  Since the git workflow issues alluded to in my
	prior commit message can be worked around, set history.immutable
	once again.

.arcconfig

@@ -1,4 +1,5 @@
 {
 	"project.name": "S",
-	"phabricator.uri" : "https://reviews.freebsd.org/"
+	"phabricator.uri" : "https://reviews.freebsd.org/",
+	"history.immutable" : true
 }