patch 9.1.0456: Left shift is incorrect with vartabstop and shiftwidth=0

Problem:  Left shift is incorrect with vartabstop and shiftwidth=0
Solution: make tabstop_at() function aware of shift direction
          (Gary Johnson)

The problem was that with 'vartabstop' set and 'shiftwidth' equal 0,
left shifts using << were shifting the line to the wrong column.  The
tabstop to the right of the first character in the line was being used
as the shift amount instead of the tabstop to the left of that first
character.

The reason was that the tabstop_at() function always returned the value
of the tabstop to the right of the given column and was not accounting
for the direction of the shift.

The solution was to make tabstop_at() aware of the direction of the
shift and to choose the tabtop accordingly.

A test was added to check this behavior and make sure it doesn't
regress.

While at it, also fix a few indentation/alignment issues.

fixes: #14864
closes: #14887

Signed-off-by: Gary Johnson <garyjohn@spocom.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/version.c

@@ -704,6 +704,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    456,
 /**/
     455,
 /**/