log-tree: make ref_filter_match() a helper method

log-tree: make ref_filter_match() a helper method

The ref_filter_match() method is defined in refs.h and implemented
in refs.c, but is only used by add_ref_decoration() in log-tree.c.
Move it into that file as a static helper method. The
match_ref_pattern() comes along for the ride.

While moving the code, also make a slight adjustment to have
ref_filter_match() take a struct decoration_filter pointer instead
of multiple string lists. This is non-functional, but will make a
later change be much cleaner.

The diff is easier to parse when using the --color-moved option.

Reported-by: Junio C Hamano <gister@pobox.com>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>