[Bug] Crashes When Clicking "Rescan Repositories in Default Clone Dir"

[ChiahongHong]

Since I wasn't sure how you would prefer to address this, I only opened an issue and didn't submit a pull request directly.

Reproduction

1. Demo directory structure

projects/
├── first/
│   └── .git/
├── second/
│   └── .git/
└── ...

2. Set the Default Clone Dir to the path where projects is located.

3. Click Rescan Repositories in Default Clone Dir.

4. SourceGit will crash and exit immediately.

Cause of the Bug

Ref: 089f24b

var normalizedRoot = rootDir.FullName.Replace("\\", "/");
var prefixLen = normalizedRoot.EndsWith('/') ? normalizedRoot.Length : normalizedRoot.Length + 1;

foreach (var f in founded)
{
    var fullpath = new DirectoryInfo(f);
    var relative = fullpath.Parent!.FullName.Replace("\\", "/").Substring(prefixLen);
    var group = FindOrCreateGroupRecursive(Preference.Instance.RepositoryNodes, relative);
    Preference.Instance.FindOrAddNodeByRepositoryPath(f, group, false);
}

1. normalizedRoot = /Users/chiahong/Documents/projects
2. prefixLen = normalizedRoot.Length + 1 = 35
3. fullpath = /Users/chiahong/Documents/projects/first
4. fullpath.Parent!.FullName = /Users/chiahong/Documents/projects
5. .Substring(35) --> index larger than the length of the string