libgit2sharp leaves _git2_ junction directories behind

[AArnott]

#393 updated libgit2sharp to a version that's buggy (libgit2/libgit2sharp#1726). We should either revert that PR, downgrade to some intermediate version that fixes worktrees but doesn't have this junction bug, or assist the libgit2sharp project to fix their bug and then update again from this project.

[AArnott]

Re-opening since #478 and other tests demonstrate this is once again a problem, and we actually downgraded libgit2sharp after the upgrade that had fixed this issue, so I guess the issue is back.