doc: typeset short command-line options as literal

It was common in our documentation to surround short option names with
forward quotes, which renders as italic in HTML. Instead, use backquotes
which renders as monospace. This is one more step toward conformance to
Documentation/CodingGuidelines.

This was obtained with:

  perl -pi -e "s/'(-[a-z])'/\`\$1\`/g" *.txt

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/Documentation/git-mktree.txt b/Documentation/git-mktree.txt
index 5c6ebdf..c3616e7 100644
--- a/Documentation/git-mktree.txt
+++ b/Documentation/git-mktree.txt
@@ -32,7 +32,7 @@
 --batch::
 	Allow building of more than one tree object before exiting.  Each
 	tree is separated by as single blank line. The final new-line is
-	optional.  Note - if the '-z' option is used, lines are terminated
+	optional.  Note - if the `-z` option is used, lines are terminated
 	with NUL.
 
 GIT