graph: fix extra spaces in graph_padding_line

graph: fix extra spaces in graph_padding_line

The graph_padding_line() function outputs a series of "|"
columns, and then pads with spaces to graph->width by
calling graph_pad_horizontally(). However, we tell the
latter that we wrote graph->num_columns characters, which is
not true; we also needed spaces between the columns. Let's
keep a count of how many characters we've written, which is
what all the other callers of graph_pad_horizontally() do.

Without this, any output that is written at the end of a
padding line will be bumped out by at least an extra
graph->num_columns spaces. Presumably nobody ever noticed
the bug because there's no code path that actually writes to
the end of a padding line.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>