Fix update commit for non-HEAD kegs with head spec (#644)

If we try to call `Formulary.from_keg(f, :head)` on the keg that
is not HEAD-keg itself, we don't need to update commit of
returned formula and should use just HEAD version with nil commit.

Same is true for `ARGV.resolved_formulae`

Library/Homebrew/extend/ARGV.rb

@@ -34,7 +34,7 @@ module HomebrewArgvExtension
           f.build = tab
           if f.head? && tab.tabfile
             k = Keg.new(tab.tabfile.parent)
-            f.version.update_commit(k.version.version.commit)
+            f.version.update_commit(k.version.version.commit) if k.version.head?
           end
         end
         f

Library/Homebrew/formulary.rb

@@ -246,7 +246,7 @@ class Formulary
       end
     end
     f.build = tab
-    f.version.update_commit(keg.version.version.commit) if f.head?
+    f.version.update_commit(keg.version.version.commit) if f.head? && keg.version.head?
     f
   end