Consider using SafeHandle in Libgit2Object instead of void*

We've had a couple of bug reports against dotnet/runtime with rarely-reproduceable crashes coming from libgit2sharp. We believe that one possible reason could be an interop anti-pattern in Libgit2Object - void*/IntPtr representation of a native handle that can be freed in finalizer. E.g. in this case the finalizer in Libgit2Object may end up calling native free here (and other overloads).

The reason why it's called an anti-pattern can be explained by a short repro in this issue: https://github.com/dotnet/runtime/issues/103522 and a general solution is to use SafeHandle for such handles. Also, see https://learn.microsoft.com/en-us/dotnet/standard/native-interop/best-practices

Reproduction steps

Expected behavior

Actual behavior

Version of LibGit2Sharp (release number or SHA1)

Operating system(s) tested; .NET runtime tested

從 LibGit2Sharp/Core/Handles/Libgit2Object.cs 第 88 行附近開始，接著追蹤 LibGit2Sharp/Core/Handles/Objects.cs 第 502-505 行附近的 finalizer 和 native-free overloads。將這些 ownership 路徑與 SafeHandle 指南及連結的 runtime 報告進行比較。完成的標準是：相關的 native handle 生命週期路徑得到安全表示並被釋放，且不會出現報告中的 finalizer race。