As of #6246, rust-analyzer follows symlinks. This can introduce an
infinite loop if symlinks point to parent directories.
Considering that #6246 was added in 2020 without many bug reports,
this is clearly a rare occurrence. However, I am observing
rust-analyzer hang on projects that have symlinks of the form:
```
test/a_symlink -> ../../
```
Ignore symlinks that only point to the parent directories, as this is
more robust but still allows typical symlink usage patterns.
bd133eecda