*Slice: don't mutate the underlying default value

If the default value is present, we shouldn't mutate it. Appending slices
as the code was before mutates the underlying value:

package main

import (
	"fmt"
)

func main() {
	slice := []string{"a", "b", "c"}

	fmt.Println(append(slice[:1], slice[2:]...))
	fmt.Println(slice)
}
~ go run tester2.go
[a c]
[a c c]

Let's fix this by creating and returning an entirely new slice.

Closes #1169

Signed-off-by: Tycho Andersen <tycho@tycho.ws>

flag_int64_slice.go

@@ -171,7 +171,9 @@ func lookupInt64Slice(name string, set *flag.FlagSet) []int64 {
 func removeFromInt64Slice(slice []int64, val int64) []int64 {
 	for i, v := range slice {
 		if v == val {
-			return append(slice[:i], slice[i+1:]...)
+			ret := append([]int64{}, slice[:i]...)
+			ret = append(ret, slice[i+1:]...)
+			return ret
 		}
 	}
 	return slice

flag_int_slice.go

@@ -170,7 +170,9 @@ func lookupIntSlice(name string, set *flag.FlagSet) []int {
 func removeFromIntSlice(slice []int, val int) []int {
 	for i, v := range slice {
 		if v == val {
-			return append(slice[:i], slice[i+1:]...)
+			ret := append([]int{}, slice[:i]...)
+			ret = append(ret, slice[i+1:]...)
+			return ret
 		}
 	}
 	return slice

flag_string_slice.go

@@ -156,7 +156,9 @@ func lookupStringSlice(name string, set *flag.FlagSet) []string {
 func removeFromStringSlice(slice []string, val string) []string {
 	for i, v := range slice {
 		if v == val {
-			return append(slice[:i], slice[i+1:]...)
+			ret := append([]string{}, slice[:i]...)
+			ret = append(ret, slice[i+1:]...)
+			return ret
 		}
 	}
 	return slice