log-tree: use ref_namespaces instead of if/else-if

log-tree: use ref_namespaces instead of if/else-if

The add_ref_decoration() method uses an if/else-if chain to determine if
a ref matches a known ref namespace that has a special decoration
category. That decoration type is later used to assign a color when
writing to stdout.

The newly-added ref_namespaces array contains all namespaces, along
with information about their decoration type. Check this array instead
of this if/else-if chain. This reduces our dependency on string literals
being embedded in the decoration logic.

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