doc: convert AsciiDoc {?foo} to ifdef::foo[]

The former seems to just be syntactic sugar for the latter.
And as it's sugar that AsciiDoctor doesn't understand, it
would be nice to avoid it. Since there are only two spots,
and the resulting source is not significantly harder to
read, it's worth doing.

Note that this does slightly affect the generated HTML (it
has an extra newline), but the rendered result for both HTML
and docbook should be the same (since the newline is not
syntactically significant there).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/diff-options.txt

@@ -23,7 +23,9 @@ ifndef::git-format-patch[]
 -u::
 --patch::
 	Generate patch (see section on generating patches).
-	{git-diff? This is the default.}
+ifdef::git-diff[]
+	This is the default.
+endif::git-diff[]
 endif::git-format-patch[]
 
 -s::
@@ -42,7 +44,9 @@ endif::git-format-patch[]
 ifndef::git-format-patch[]
 --raw::
 	Generate the raw format.
-	{git-diff-core? This is the default.}
+ifdef::git-diff-core[]
+	This is the default.
+endif::git-diff-core[]
 endif::git-format-patch[]
 
 ifndef::git-format-patch[]