doc: typeset long command-line options as literal

Similarly to the previous commit, use backquotes instead of
forward-quotes, for long options.

This was obtained with:

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

and manual tweak to remove false positive in ascii-art (o'--o'--o' to
describe rewritten history).

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/gitdiffcore.txt

@@ -28,8 +28,8 @@ The 'git diff-{asterisk}' family works by first comparing two sets of
 files:
 
  - 'git diff-index' compares contents of a "tree" object and the
-   working directory (when '--cached' flag is not used) or a
-   "tree" object and the index file (when '--cached' flag is
+   working directory (when `--cached` flag is not used) or a
+   "tree" object and the index file (when `--cached` flag is
    used);
 
  - 'git diff-files' compares contents of the index file and the