log: add --clear-decorations option

log: add --clear-decorations option

The previous changes introduced a new default ref filter for decorations
in the 'git log' command. This can be overridden using
--decorate-refs=HEAD and --decorate-refs=refs/, but that is cumbersome
for users.

Instead, add a --clear-decorations option that resets all previous
filters to a blank filter that accepts all refs.

Signed-off-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>