Add .mm as an XML extension with heuristic rule

lib/linguist/heuristics.rb

@@ -34,6 +34,9 @@ module Linguist
         if languages.all? { |l| ["AsciiDoc", "AGS Script"].include?(l) }
           result = disambiguate_asc(data, languages)
         end
+        if languages.all? { |l| ["XML", "Objective-C++"].include?(l) }
+          result = disambiguate_mm(data, languages)
+        end
         return result
       end
     end
@@ -124,6 +127,16 @@ module Linguist
       matches
     end
 
+    def self.disambiguate_mm(data, languages)
+      matches = []
+      if /<map version="[^"]+">/.match(data)
+        matches << Language["XML"]
+      else
+        matches << Language["Objective-C++"]
+      end
+      matches
+    end
+
     def self.active?
       !!ACTIVE
     end

lib/linguist/languages.yml

@@ -2771,6 +2771,7 @@ XML:
   - .jelly
   - .kml
   - .launch
+  - .mm
   - .mxml
   - .nproj
   - .nuspec