doc: git-diff: apply new documentation guidelines

doc: git-diff: apply new documentation guidelines

The documentation for git-diff has been updated to follow the new
documentation guidelines. The following changes have been applied to
the series of patches:

- switching the synopsis to a synopsis block which will automatically
  format placeholders in italics and keywords in monospace
- use _<placeholder>_ instead of <placeholder> in the description
- use `backticks for keywords and more complex option
descriptions`. The new rendering engine will apply synopsis rules to
these spans.
- prevent git-diff from self-referencing itself via gitlink macro when
the generated link would point to the same page.

Signed-off-by: Jean-Noël Avila <jn.avila@free.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>