gitk: use config settings for head/tag colors

gitk: use config settings for head/tag colors

The drawtags procedure currently uses headfgcolor for all label text,
ignoring the tagfgcolor setting.

The call to create the outline polygon for (non-tag) heads currently
has the color for headoutlinecolor hardcoded to black.

This patch maintains the variables for the non-tag refs so that heads
are colored differently from non-head (non-tag) refs.

The outline and fill colors for the non-head refs remain hardcoded to
the prior values, black & #ddddff.

Signed-off-by: Shannon Barber <sgbarber@gmail.com>
Signed-off-by: Johannes Sixt <j6t@kdbg.org>