shx cp -r Creates nested recursive directories

There's a difference between the bash cp -r behavior and shx cp -r when trying to copy a directory into itself:

Create a directory ./dist with a single file in it (e.g., "test.txt") and run:

1. (in bash) cp -ur "./dist" "./dist/prod"
2. (with npm) "shx cp -ur "./dist" "./dist/prod""

Test 1. will result in a message saying "cp: cannot copy a directory, './dist/', into itself, './dist/prod'" but test 2. will result in a nested directory structure with a large amount of layers, each ./dist/prod subdirectory will contain another "/prod" subdirectory recursively.

Using shx version "0.3.4"

Reproduce the issue with bash cp -ur and shx cp -ur using ./dist and ./dist/prod, then trace shx's copy-command entry point. Compare how each handles copying a directory into itself. Done means shx reports the self-copy error instead of creating nested prod directories.