decorate: color each token separately

decorate: color each token separately

Wrap "tag:" prefixes and the arrows in "HEAD -> branch" decorations in
their own color sequences. Otherwise, if --graph is used, tag names or
arrows can end up uncolored when %w width formatting breaks a line just
before them. This is because --graph resets the color after doing its
drawing at the start of a line.

Amend test t4207-log-decoration-colors.sh accordingly.

Signed-off-by: Andy Koppe <andy.koppe@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>