doc: convert git-mergetool manpage to new synopsis 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.
Signed-off-by: Jean-Noël Avila <redacted> Signed-off-by: Junio C Hamano <redacted>