An error in #1202 build led us to discover we're missing a GitErrorCode entry mapping from GIT_EDIRECTORY. Let's fix this.

References:

• https://github.com/libgit2/libgit2/blob/master/include/git2/errors.h
• https://github.com/libgit2/libgit2sharp/blob/vNext/LibGit2Sharp/Core/GitErrorCode.cs