Merge pull request #218 from oranja/bugfix-git-ahead

bugfix: add missing $s in variable tests in git_ahead.fish

lib/git/git_ahead.fish

 function git_ahead -a ahead behind diverged none
   git_is_repo; and begin
     test -z "$ahead"; and set ahead "+"
-    test -z "behind"; and set behind "-"
-    test -z "diverged"; and set diverged "±"
-    test -z "none"; and set none ""
+    test -z "$behind"; and set behind "-"
+    test -z "$diverged"; and set diverged "±"
+    test -z "$none"; and set none ""
     command git rev-list --left-right "@{upstream}...HEAD" ^/dev/null \
     | awk "/>/ {a += 1} /</ {b += 1} \
       {if (a > 0) nextfile} END \