Encodings::fromLaTeXCommand: if the command directly maps an entry of unicodesymbols, use it and bypass most of the logic.

This is important for commands like !`, that are equivalent to \textexclamdown. However, ! is matched earlier, because the logic works with prefixes, hence the output doesn't make sense.

autotests/export/docbook/ert_convert.xml

@@ -3,5 +3,6 @@
   See https://www.lyx.org/ for more information -->
 <article xml:lang="en_US" xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:m="http://www.w3.org/1998/Math/MathML" xmlns:xi="http://www.w3.org/2001/XInclude" version="5.2">
 <title>ERT Conversions</title>
-<para>These should be <code>&amp;#192;</code>: &#192; &#192; &#192; </para>
+<para>These should be <code>&amp;#192;</code>: &#192; &#192; &#192;</para>
+<para>This one should be <code>&amp;#161;</code>: &#161; &#161;</para>
 </article>

src/Encoding.cpp

@@ -381,6 +381,30 @@ docstring Encodings::fromLaTeXCommand(docstring const & cmd, int cmdtype,
 	rem = empty_docstring();
 	bool const mathmode = cmdtype & MATH_CMD;
 	bool const textmode = cmdtype & TEXT_CMD;
+
+	// Easy case: the command is a complete entry of unicodesymbols.
+	for (const auto & unicodeSymbol : unicodesymbols) {
+		if (mathmode) {
+			for (const auto & command : unicodeSymbol.second.mathCommands()) {
+				if (command == cmd) {
+					docstring value;
+					value += unicodeSymbol.first;
+					return value;
+				}
+			}
+		}
+		if (textmode) {
+			for (const auto & command : unicodeSymbol.second.textCommands()) {
+				if (command == cmd) {
+					docstring value;
+					value += unicodeSymbol.first;
+					return value;
+				}
+			}
+		}
+	}
+
+	// Otherwise, try to map as many commands as possible, matching prefixes of the command.
 	docstring symbols;
 	size_t const cmdend = cmd.size();
 	size_t prefix = 0;