Bug report

Bug summary
If I two paths has a common segment, the result of a.intersects_path(b) is inconsistent. I think it should return True every time in this case.

Code for reproduction

from matplotlib.path import Path

a = Path([(0,1),(0,3)]) 
b = Path([(0,0),(0,4)])   # larger than a
c = Path([(0,0),(0,2)])   # ends in a
d = Path([(0,1),(0,2),(0,3)])   # same as a, but has an extra point in a
e = Path([(0,0),(0,2),(0,4)])   # same as b, but has an extra point in a
f = Path([(0,0),(0,1),(0,3),(0,4)])  # contains a as a segment
g = Path([(0,1),(0,3),(1,3)])   # a and a non-collinear point

paths = [a,b,c,d,e,f, g]
for i in paths:
    print([1 if p.intersects_path(i) else 0 for p in paths])

Actual outcome

[0, 0, 0, 0, 1, 1, 1]
[0, 0, 0, 0, 0, 0, 1]
[0, 0, 0, 0, 0, 0, 0]
[0, 0, 0, 0, 1, 1, 1]
[1, 0, 0, 1, 0, 0, 1]
[1, 0, 0, 1, 0, 0, 1]
[1, 1, 0, 1, 1, 1, 1]

Expected outcome
True for all (since they all have the [(0,1),(0,2)] segment in common).

Matplotlib version

• Operating system: Ubuntu 16.04
• Matplotlib version: 2.2.2
• Matplotlib backend (print(matplotlib.get_backend())): module://ipykernel.pylab.backend_inline
• Python version: 3.6.5
• Jupyter version (if applicable): 5.4.1