With a certain topology involving an octopus merge, git log --graph
--oneline --all --color=never produces output which includes some ANSI
escape code coloring. Attached is a script to reproduce the problem
(creates a git repository in subdir log-format-test), along with
sample graph and valgrind output (indicates some unitialialized memory
access).

I've observed the problem with Windows git versions 2.7.0, 2.5.3.
I've NOT observed it with 1.9.5,

On GNU/Linux the symptom only appears when running with valgrind, I
tried versions
2.8.0, and 2.8.2.402.gedec370 (the last is where the valgrind output comes from)