Adds /bin to the end of the path unnecessarily

Recently, Arch Linux made a change where /bin and /sbin are now symlinked to /usr/bin. As a result, only /usr/bin is added to the PATH. fish then sees this PATH, freaks out that it doesn't have /bin, and adds /bin to the end, which is kind of odd.

[zanchey]

I'm not against making the change suggested in #854, but can you explain what the current behaviour breaks? As far as I can see, the current code appends /bin to the existing path, and any overrides from ccache or similar should be prepended.

(Lots of OSes have /bin as a symlink to /usr/bin.)

...I actually just assumed that they broke and didn't check. You are correct; they don't break. I still agree with the change, though.

[ridiculousfish]

In 85ce80d, I made setup_path not try to mess with the PATH unless it's entirely empty. I believe that should resolve this. Thanks for reporting this!