LaTeX writer: change braced backtick to \textasciigrave{}

Backticks in verbatim environments are converted to
open-single-quotes. This change makes them appear as backticks. This
corresponds to how we treat `'' in verbatim environments (with
\textquotesingle{}).

src/Text/Pandoc/Writers/LaTeX.hs

@@ -276,7 +276,7 @@ stringToLaTeX  ctx (x:xs) = do
        '€' -> "\\euro{}" ++ rest
        '{' -> "\\{" ++ rest
        '}' -> "\\}" ++ rest
-       '`' | ctx == CodeString -> "{`}" ++ rest
+       '`' | ctx == CodeString -> "\\textasciigrave{}" ++ rest
        '$' | not isUrl -> "\\$" ++ rest
        '%' -> "\\%" ++ rest
        '&' -> "\\&" ++ rest

tests/Tests/Writers/LaTeX.hs

@@ -76,6 +76,6 @@ tests = [ testGroup "code blocks"
           , "single quotes" =:
               code "dog's" =?> "\\texttt{dog\\textquotesingle{}s}"
           , "backtick" =:
-              code "`nu?`" =?> "\\texttt{{`}nu?{`}}"
+              code "`nu?`" =?> "\\texttt{\\textasciigrave{}nu?\\textasciigrave{}}"
           ]
         ]