doc: convert git-mv to new documentation format

doc: convert git-mv to new documentation format

- Switch 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.

Unfortunately, there's an inconsistency in the synopsis style, where
the ellipsis is used to indicate that the option can be repeated, but
it can also be used in Git's three-dot notation to indicate a range of
commits. The rendering engine will not be able to distinguish
between these two cases.

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