internal/flags: fix issue with stringslice migration

internal/flags/helpers.go

@@ -92,10 +92,34 @@ func MigrateGlobalFlags(ctx *cli.Context) {
 }
 
 func doMigrateFlags(ctx *cli.Context) {
+	// Figure out if there are any aliases of commands. If there are, we want
+	// to ignore them when iterating over the flags.
+	var aliases = make(map[string]bool)
+	for _, fl := range ctx.Command.Flags {
+		for _, alias := range fl.Names()[1:] {
+			aliases[alias] = true
+		}
+	}
 	for _, name := range ctx.FlagNames() {
 		for _, parent := range ctx.Lineage()[1:] {
 			if parent.IsSet(name) {
-				ctx.Set(name, parent.String(name))
+				// When iterating across the lineage, we will be served both
+				// the 'canon' and alias formats of all commmands. In most cases,
+				// it's fine to set it in the ctx multiple times (one for each
+				// name), however, the Slice-flags are not fine.
+				// The slice-flags accumulate, so if we set it once as
+				// "foo" and once as alias "F", then both will be present in the slice.
+				if _, isAlias := aliases[name]; isAlias {
+					continue
+				}
+				// If it is a string-slice, we need to set it as
+				// "alfa, beta, gamma" instead of "[alfa beta gamma]", in order
+				// for the backing StringSlice to parse it properly.
+				if result := parent.StringSlice(name); len(result) > 0 {
+					ctx.Set(name, strings.Join(result, ","))
+				} else {
+					ctx.Set(name, parent.String(name))
+				}
 				break
 			}
 		}