Texinfo writer: fix escaping in links.

Previously we dropped periods, commas, etc. This was a regression
from 2024. All we really need to do is escape the comma.

Closes #11758.

test/command/11758.md

@@ -0,0 +1,13 @@
+```
+% pandoc -t texinfo
+[a.b](url)
+
+[a,b](url)
+^D
+@node Top
+@top Top
+
+@uref{url,a.b}
+
+@uref{url,a@comma{}b}
+```

test/writer.texinfo

@@ -948,7 +948,7 @@ In a list?
 It should.
 @end itemize
 
-An e-mail address: @uref{mailto:nobody@@nowhere.net,nobody@@nowherenet}
+An e-mail address: @uref{mailto:nobody@@nowhere.net,nobody@@nowhere.net}
 
 @quotation
 Blockquoted: @url{http://example.com/}