doc: apply new format to git-branch man page

doc: apply new format to git-branch man page

- Switch the synopsis to a synopsis block which automatically
  formats 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 applies synopsis rules to
these spans.

Possible values for some variables, that were mentioned in the description
prose, are now made into enumerated list.

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