sideband: color lines with keyword only

sideband: color lines with keyword only

When bf1a11f0a1 (sideband: highlight keywords in remote sideband output,
2018-08-07) was introduced, it was carefully considered which strings
would be highlighted. However 59a255aef0 (sideband: do not read beyond
the end of input, 2018-08-18) brought in a regression that the original
did not test for. A line containing only the keyword and nothing else
("SUCCESS") should still be colored.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>