git-show-ref: fix escaping in asciidoc source

Two "^" characters were incorrectly being interpreted as markup for
superscripting.  Fix them by writing them as attribute references
"{caret}".

Although a single "^" character in a paragraph cannot be
misinterpreted in this way, also write other "^" characters as
"{caret}" in the interest of good hygiene (unless they are in literal
paragraphs, of course, in which context attribute references are not
recognized).

Spell "{}" consistently, namely *not* quoted as "\{\}".  Since the
braces are empty, they cannot be interpreted as an attribute
reference, and either spelling is OK.  So arbitrarily choose one
variation and use it consistently.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-show-ref.txt

@@ -44,7 +44,7 @@ OPTIONS
 -d::
 --dereference::
 
-	Dereference tags into object IDs as well. They will be shown with "^{}"
+	Dereference tags into object IDs as well. They will be shown with "{caret}{}"
 	appended.
 
 -s::
@@ -73,9 +73,9 @@ OPTIONS
 --exclude-existing[=<pattern>]::
 
 	Make 'git show-ref' act as a filter that reads refs from stdin of the
-	form "^(?:<anything>\s)?<refname>(?:{backslash}{caret}\{\})?$"
+	form "{caret}(?:<anything>\s)?<refname>(?:{backslash}{caret}{})?$"
 	and performs the following actions on each:
-	(1) strip "^{}" at the end of line if any;
+	(1) strip "{caret}{}" 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;
 	(4) ignore if refname is a ref that exists in the local repository;