utils: merkletrie, filesystem fix symlinks to dir

[dimonomid]

I tried to figure how to write a proper test, but failed in a reasonable time frame, so sending the PR without a test. Nevertheless, here is a tiny repository which reproduces an issue: https://github.com/dimonomid/git-go-symlink-test/blob/master/test_symlink_dir.go

It uses another repository as a test case: https://github.com/dimonomid/git-go-fixture-dir-symlink , which has just two commits; the second one adds this tree:

.
├── dir2
│   └── file2
└── dir3
    └── dir2_symlink -> ../dir2

And to reproduce the issue, we can just clone the repo (so that we have these dir2/file2 and symlink to it), and then checkout the previous revision. This is the list of changes it tries to apply:

<Delete dir2/file2>
<Delete dir3/dir2_symlink>
<Delete dir3/dir2_symlink/file2>

And obviously the last one fails, because dir2/file2 was already removed before. This PR prevents non-dir items from having children.

[mcuadros]

Make sense, but we need a test for merge this.