src/post2html: Disable TeX-style tie parsing in fences

Code fences are meant to indicate literal text, not formatted.

src/post2html

@@ -98,11 +98,11 @@ prefmt()
     }
 
     # ties
-    { $0 = gensub( /([^\\])~/, "\\1 ", "g" ) }
-    { $0 = gensub( /\\~/, "~", "g" ) }
+    !infence { $0 = gensub( /([^\\])~/, "\\1 ", "g" ) }
+    !infence { $0 = gensub( /\\~/, "~", "g" ) }
 
     # TeX-style newline removal
-    /%$/ {
+    !infence && /%$/ {
       gsub( /%$/, "" )
       printf "%s", $0
       triml = 1
@@ -113,6 +113,7 @@ prefmt()
     # code block is HTML and we want to render it as an example)
     /^```/ {
       gather = !gather
+      infence = gather
       if ( gather ) gblock = ""
     }
     !/^```/ && gather { gblock = gblock $0 "\n" }