path: Make path real "work" with nonexistent paths

This just goes back until it finds an existent path, resolves that,
and adds the normalized rest on top.

So if you try

/bin/foo/bar////../baz

and /bin exists as a symlink to /usr/bin, it would resolve that, and
normalize the rest, giving

/usr/bin/foo/baz

(note: We might want to add this to realpath as well?)

tests/checks/path.fish

@@ -110,3 +110,14 @@ path real bin//sh | string match -r -- 'bin/bash$'
 # The "//" is squashed, and the symlink is resolved.
 # sh here is bash
 # CHECK: bin/bash
+
+# `path real` with nonexistent paths
+set -l path (path real foo/bar)
+string match -rq "^"(string escape --style=regex -- $PWD)'/' -- $path
+and echo It matches pwd!
+# CHECK: It matches pwd!
+string replace -r "^"(string escape --style=regex -- $PWD)'/' "" -- $path
+# CHECK: foo/bar
+
+path real /banana//terracota/terracota/booooo/../pie
+# CHECK: /banana/terracota/terracota/pie