Revert "Implement basename in shell"

This reverts commit 794f0aac.

Conflicts:
	vcsh

Turns out the shell implementation didn't account for trailing
slashes. While, arguably, a proper interactive shell should eat
them before execution anyway, this can also be called in script
context.

This means that

    vcsh clone foo/

made it fail and even after commit 7e33e38a

    vcsh clone foo//

would have failed.

Revert to safe and time-proven default: Standalone `basename`.

For further information, see:

https://github.com/RichiH/vcsh/pull/92