Avoid target directories within the source directory getting stuck.

[Fleuv]

Currently when you run:

$filesystem->mirror('/foo', '/foo/bar');

The method will get caught in a recursive loop of creating "bar" sub directories in the target directory.

This will happen:

/foo/bar/bar/
/foo/bar/bar/bar/
/foo/bar/bar/bar/bar/
...

Until the path is so long that is validated as an invalid path string and thus mkdir() crashes.

In this pull request I implemented an if statement at the beginning of the foreach in the mirror() method. This if statement ensures that the target directory in the mirror process is skipped.

[symfony-bot]

Thanks for your pull request! We love contributions.

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