fish_add_path: Don't resolve symlinks

The case for symlinked directories being duplicated a lot isn't there,
but there *is* a usecase for adding the symlink rather than the
target, and that's homebrew.

E.g. homebrew installs ruby into /usr/local/Cellar/ruby/2.7.1_2/bin,
and links to it from /usr/local/opt/ruby/bin. If we add the target, we
would miss updates.

Having path entries that point to the same location isn't a big
problem - it's a path lookup, so it takes a teensy bit longer. The
canonicalization is mainly so paths don't end up duplicated via weird
spelling and so relative paths can be used.

tests/checks/fish_add_path.fish

@@ -30,19 +30,20 @@ echo $status
 # CHECK: 1
 functions --erase checkpath
 
-# Not adding a link either
+# Add a link to the same path.
 fish_add_path -v $tmpdir/link
+# CHECK: set fish_user_paths {{.*}}/link {{.*}}/bin
 echo $status
-# CHECK: 1
+# CHECK: 0
 
 fish_add_path -a $tmpdir/sbin
 # Not printing anything because it's not verbose, the /sbin should be added at the end.
 string replace -- $tmpdir '' $fish_user_paths | string join ' '
-# CHECK: /bin /sbin
+# CHECK: /link /bin /sbin
 
 fish_add_path -m $tmpdir/sbin
 string replace -- $tmpdir '' $fish_user_paths | string join ' '
-# CHECK: /sbin /bin
+# CHECK: /sbin /link /bin
 
 set -l oldpath "$PATH"
 fish_add_path -nP $tmpdir/etc | string replace -- $tmpdir ''