doc: convert git-merge manpage to new style

doc: convert git-merge manpage to new style

- 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.

In order to avoid breaking the format on '<<<<<<' and '>>>>>' lines
by applying the synopsis rules to these spans, they are formatted using '+'
signs instead of '`' signs.

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