Allow fish comp to support trailing empty lines

Some programs may output extra empty lines after the directive.
Those lines must be ignored for fish shell completion to work.
zsh and bash are not impacted.

Signed-off-by: Marc Khouzam <marc.khouzam@montreal.ca>
Co-authored-by: Johannes Altmanninger <aclopte@gmail.com>

fish_completions.go

@@ -53,6 +53,19 @@ function __%[1]s_perform_completion
     __%[1]s_debug "Calling $requestComp"
 
     set results (eval $requestComp 2> /dev/null)
+
+    # Some programs may output extra empty lines after the directive.
+    # Let's ignore them or else it will break completion.
+    # Ref: https://github.com/spf13/cobra/issues/1279
+    for line in $results[-1..1]
+        if test (string trim -- $line) = ""
+            # Found an empty line, remove it
+            set results $results[1..-2]
+        else
+            # Found non-empty line, we have our proper output
+            break
+        end
+    end
     set comps $results[1..-2]
     set directiveLine $results[-1]