Escape starting periods in ms writer code blocks

If a line of ms code block output starts with a period (.), it should
be prepended by '\&' so that it is not interpreted as a roff command.

Fixes #6505

pandoc.cabal

@@ -816,6 +816,7 @@ test-suite test-pandoc
                   Tests.Writers.FB2
                   Tests.Writers.Powerpoint
                   Tests.Writers.OOXML
+                  Tests.Writers.Ms
   if os(windows)
     cpp-options:      -D_WINDOWS
   default-language: Haskell2010

src/Text/Pandoc/Writers/Ms.hs

@@ -204,7 +204,9 @@ blockToMs opts (CodeBlock attr str) = do
     literal ".IP" $$
     literal ".nf" $$
     literal "\\f[C]" $$
-    hlCode $$
+    ((case T.uncons str of
+      Just ('.',_) -> literal "\\&"
+      _            -> mempty) <> hlCode) $$
     literal "\\f[]" $$
     literal ".fi"
 blockToMs opts (LineBlock ls) = do

test/Tests/Writers/Ms.hs

@@ -0,0 +1,37 @@
+{-# LANGUAGE NoImplicitPrelude #-}
+{-# LANGUAGE OverloadedStrings #-}
+module Tests.Writers.Ms (tests) where
+
+import Prelude
+import Test.Tasty
+import Tests.Helpers
+import Text.Pandoc
+import Text.Pandoc.Builder
+
+infix 4 =:
+(=:) :: (ToString a, ToPandoc a)
+     => String -> (a, String) -> TestTree
+(=:) = test (purely (writeMs def . toPandoc))
+
+tests :: [TestTree]
+tests = [ testGroup "code blocks"
+          [ "basic"
+              =: codeBlock "hello"
+              =?> unlines
+              [ ".IP"
+              , ".nf"
+              , "\\f[C]"
+              , "hello"
+              , "\\f[]"
+              , ".fi"]
+          , "escape starting ."
+              =: codeBlock ". hello"
+              =?> unlines
+              [ ".IP"
+              , ".nf"
+              , "\\f[C]"
+              , "\\&. hello"
+              , "\\f[]"
+              , ".fi"]
+          ]
+        ]

test/test-pandoc.hs

@@ -37,6 +37,7 @@ import qualified Tests.Writers.JATS
 import qualified Tests.Writers.Jira
 import qualified Tests.Writers.LaTeX
 import qualified Tests.Writers.Markdown
+import qualified Tests.Writers.Ms
 import qualified Tests.Writers.Muse
 import qualified Tests.Writers.Native
 import qualified Tests.Writers.Org
@@ -70,6 +71,7 @@ tests pandocPath = testGroup "pandoc tests"
           , testGroup "Muse" Tests.Writers.Muse.tests
           , testGroup "FB2" Tests.Writers.FB2.tests
           , testGroup "PowerPoint" Tests.Writers.Powerpoint.tests
+          , testGroup "Ms" Tests.Writers.Ms.tests
           ]
         , testGroup "Readers"
           [ testGroup "LaTeX" Tests.Readers.LaTeX.tests