doc: convert git-log to new documentation format

doc: convert git-log 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.

We also transform inline descriptions of possible values of option
--decorate into a list, which is more readable and extensible.

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