ls: Don't reject multiple use of --color

[sylvestre]

./target/debug/coreutils ls --color=auto --color=always   
error: the argument '--color[=<color>]' cannot be used multiple times

Usage: ./target/debug/coreutils ls [OPTION]... [FILE]...

GNU accepts it

[rabbull]

Hi,

Can I propose a PR to this issue? My plan is to set parsing action of color argument to Append, which allows clap to parse multiple duplicating tags. After that, when extract the option, select only the last color tags. In addition, a test case for multiple use of --color will be added.

Best,
rabbull

[cakebaker]

#5641 already resolves this issue as a side effect by setting args_override_self(true)