Better LibGit2Sharp Commits

In #534, git-tfs started using libgit2sharp to create commits. A side effect of that were these behavioral changes:

* commit message line endings changed from `\r\n` to `\n`
* the dates on commits became the UTC time with the local tz offset

This branch:

* uses a new API by @amibar (libgit2/libgit2sharp#611) to preserve TFS's line endings in commit messages. 
* fixes the dates on commit messages.

cc @pmiossec @KindDragon

Closes #533