doc: allow literal and emphasis format in doc vs help tests

As the new formatting of literal and placeholders is introduced,
the synopsis in the man pages can now hold additional markup with
respect to the command help.

Signed-off-by: Jean-Noël Avila <jn.avila@free.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t0450-txt-doc-vs-help.sh

@@ -59,7 +59,9 @@ txt_to_synopsis () {
 		-e '/^\[verse\]$/,/^$/ {
 			/^$/d;
 			/^\[verse\]$/d;
-
+			s/_//g;
+			s/++//g;
+			s/`//g;
 			s/{litdd}/--/g;
 			s/'\''\(git[ a-z-]*\)'\''/\1/g;