Documentation: unbreak regex in show-ref manual

I am not sure why, but the regular expression "(?:\^\{\})" gets
rendered by asciidoc as "(?:\{})".  The intent seems to be a regex
matching the literal string "^{}", so this rewrites the markup to
produce "(?:\^{})" as output.

Cc: Julian Phillips <julian@quantumfyre.co.uk>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/asciidoc.conf

@@ -16,6 +16,7 @@ plus=+
 caret=^
 startsb=[
 endsb=]
+backslash=\
 tilde=~
 apostrophe='
 backtick=`

Documentation/git-show-ref.txt

@@ -73,8 +73,8 @@ OPTIONS
 --exclude-existing[=<pattern>]::
 
 	Make 'git show-ref' act as a filter that reads refs from stdin of the
-	form "^(?:<anything>\s)?<refname>(?:\^\{\})?$" and performs the
-	following actions on each:
+	form "^(?:<anything>\s)?<refname>(?:{backslash}{caret}\{\})?$"
+	and performs the following actions on each:
 	(1) strip "^{}" at the end of line if any;
 	(2) ignore if pattern is provided and does not head-match refname;
 	(3) warn if refname is not a well-formed refname and skip;