Following 'enum bisect_error' vocabulary, return variable 'res' is
always non-positive.
Let's use '-res' instead of 'abs(res)' to make the code clearer.
Mentored-by: Christian Couder <redacted> Helped-by: Junio C Hamano <redacted> Signed-off-by: Miriam Rubio <redacted> Signed-off-by: Junio C Hamano <redacted>