seq.fish: use gseq if available.

Apparently if you install gnu coreutils on OpenBSD, the tools are
g-prefixed. So we definitely want to just alias that rather than
provide our lousy shell script implementation.

share/functions/seq.fish

@@ -2,10 +2,18 @@
 # We can't call type here because that also calls seq
 
 if not command -sq seq
+    if command -sq gseq
+        # No seq provided by the OS, but GNU coreutils was apparently installed, fantastic
+        function seq --description "Print sequences of numbers (gseq)"
+            gseq $argv
+        end
+        exit
+    else
         # No seq command
         function seq --description "Print sequences of numbers"
             __fish_fallback_seq $argv
         end
+    end
 
     function __fish_fallback_seq --description "Fallback implementation of the seq command"