Documentation: use brackets for optional arguments

Documentation: use brackets for optional arguments

The documentation for git blame used vertical bars for optional
arguments to -M and -C, which is unusual and potentially confusing.
Since most man pages use brackets for optional items, and that's
consistent with how we document the same options for git diff and
friends, use brackets here, too.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>