[LinuxCNC/linuxcnc Issue#524] reverse-run-master2 acceleration issue

Here are the steps I follow to reproduce the issue:

1. Clone linuxcnc repo, checkout reverse-run-master2 branch, build lcnc and then run with axis_mm from sim/axis
2. Load this code:
;line test
G21
G64 P0.005
M52 P1
F1000
G0 X10 Y10
M3 S1
G1 X90
G1 X100 Y20
M5
G0 X0 Y0
M2

3. While running, do setp motion.adaptive-feed -1 until the control point reaches the start point then do setp motion.adaptive-feed 1, motion restarts in the forward direction but acceleration is painfully slow.
If I only do a partial reverse then acceleration appears normal (until a full reverse is applied).
If I remove line 9 (G1 X100 Y20) acceleration behaves normally.
If I use arcs, acceleration also appears normal.

This only happens in the first segment, after the second segment is begun then acceleration returns to normal.

Information about my hardware and software:

* I am using Debian GNU/Linux 9.4 (stretch)
* I am using Linux stretch 4.9.0-6-rt-amd64 #1 SMP PREEMPT RT Debian 4.9.88-1+deb9u1 (2018-05-07) x86_64 GNU/Linux
* I am running a rip version
* I am using this LinuxCNC version, v2.8.0~pre1~feature-reverse-run-master2~bef8d9d52
* I am using AXIS
* I am using axis_mm sim from the configuration selector