Add :hg build requirement.

Closes Homebrew/homebrew#19074.

Library/Homebrew/dependency_collector.rb

@@ -91,6 +91,7 @@ class DependencyCollector
     when :tex        then TeXDependency.new(tag)
     when :clt        then CLTDependency.new(tag)
     when :arch       then ArchRequirement.new(tag)
+    when :hg         then MercurialDependency.new(tag)
     else
       raise "Unsupported special dependency #{spec}"
     end

Library/Homebrew/requirements.rb

@@ -107,3 +107,17 @@ class ArchRequirement < Requirement
     "This formula requires an #{@arch} architecture."
   end
 end
+
+class MercurialDependency < Requirement
+  fatal true
+
+  satisfy { which('hg') }
+
+  def message; <<-EOS.undent
+    Mercurial is needed to install this software.
+
+    You can install this with Homebrew using:
+      brew install mercurial
+    EOS
+  end
+end