decorate: use commit color for HEAD arrow

decorate: use commit color for HEAD arrow

Use the commit color instead of the HEAD color for the arrow or custom
symbol in "HEAD -> branch" decorations, for visual consistency with the
prefix, separator and suffix symbols, which are also colored with the
commit color.

This change was triggered by the possibility that one could choose to
use the same symbol for the pointer and the separator options in
%(decorate), in which case they ought to be the same color.

A related precedent is 'ls -l', where the arrow for symlinks gets the
default color rather than that of the symlink name.

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>