column: add dense layout support

Normally all cells (and in turn columns) share the same width. This
layout mode can waste space because one long item can stretch our all
columns.

With COL_DENSE enabled, column width is calculated indepdendently. All
columns are shrunk to minimum, then it attempts to push cells of the
last row over to the next column with hope that everything still fits
even there's one row less. The process is repeated until the new layout
cannot fit in given width any more, or there's only one row left
(perfect!).

Apparently, this mode consumes more cpu than the old one, but it makes
better use of terminal space. For layouting one or two screens, cpu
usage should not be detectable.

This patch introduces option handling code besides layout modes and
enable/disable to expose this feature as "dense". The feature can be
turned off by specifying "nodense".

Thanks-to: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

column.h

@@ -4,6 +4,8 @@
 #define COL_LAYOUT_MASK   0x000F
 #define COL_ENABLE_MASK   0x0030   /* always, never or auto */
 #define COL_PARSEOPT      0x0040   /* --column is given from cmdline */
+#define COL_DENSE         0x0080   /* Shrink columns when possible,
+				      making space for more columns */
 
 #define COL_DISABLED      0x0000   /* must be zero */
 #define COL_ENABLED       0x0010