git-show-ref: fix escaping in asciidoc source

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>