Fix substitution for $$Contents.

The second argument here is the length of the string to replace,
not the position of the final character.

(cherry picked from commit 6b97f2c0751ff79fc19ea0e11b4ef957fd71324c)

src/insets/ExternalSupport.cpp

@@ -199,7 +199,7 @@ string const doSubstitution(InsetExternalParams const & params,
 
 		size_t const pos = result.find("$$Contents(\"");
 		size_t const end = result.find("\")", pos);
-		result.replace(pos, end + 2, contents);
+		result.replace(pos, end + 2- pos, contents);
 	}
 
 	return result;

status.22x

@@ -45,6 +45,8 @@ What's new
 
 - Fixed preview of external material with plaintext and DocBook.
 
+- Fixed substitution for $$Contents in external templates.
+
 
 * LYX2LYX