Commits date wrong when date is prior to 01/01/1970

When creating a commit with a signature dated before 01/01/1970,
even if the signature is correct, the commit gets a wrong date.

(If intended, is there a work-around ?)

Reproduction steps

var author = Signature(author_name, author_email, author_date)

var last_commit = repo.Commit(message, author, author)

Expected behavior

author.When == last_commit.Author.When

Actual behavior

if author_date < 01/01/1970
author.When != last_commit.Author.When

Version of LibGit2Sharp (release number or SHA1)

libgit2sharp 0.26.0

Operating system(s) tested; .NET runtime tested

dotnet 2.2.401 on Ubuntu 18.04.3 LTS

首先，在 .NET 2.2.401 和 Ubuntu 18.04.3 上，使用早于 01/01/1970 的作者日期，复现报告中 Signature 和 Commit 调用产生的问题。跟踪 commit 作者日期的序列化和读取过程，然后验证对于 Unix epoch 之前的日期，author.When equals last_commit.Author.When。