diff.c: fix a graph output bug

diff.c: fix a graph output bug

When --graph is in effect, the line-prefix typically has colored graph
line segments and ends with reset.  The color sequence "set" given to
this function is for showing the metainfo part of the patch text and
(1) it should not be applied to the graph lines, and (2) it will be
reset at the end of line_prefix so it won't be in effect anyway.

Signed-off-by: Bo Yang <redacted>
Signed-off-by: Junio C Hamano <redacted>