report: fix negative colors in barcharts

Specified bar colors only apply to positive bars in the plot.
Negative bars still have the default colors, which are slightly
darker shades of the default positive colors.

This commit forces negative bars to have the same colors as
positive bars.

gnucash/report/report-system/html-barchart.scm

@@ -514,6 +514,9 @@
                   (push "options.seriesColors = [")
                   (push colors-str)
                   (push "];\n")
+                  (push "options.negativeSeriesColors = [")
+                  (push colors-str)
+                  (push "];\n")
                   )
                 )