replace info: rename 'full' to 'long' and clarify in-code symbols

Enum names SHORT/MEDIUM/FULL were too broad to be descriptive.  And
they clashed with built-in symbols on platforms like Windows.
Clarify by giving them REPLACE_FORMAT_ prefix.

Rename 'full' format in "git replace --format=<name>" to 'long', to
match others (i.e. 'short' and 'medium').

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>