Release LibGit2Sharp v0.18.1

@kitbellew This includes the fix for the cross volumes local cloning issue (https://github.com/libgit2/libgit2/pull/2399)

@Therzok This includes the fix for the removal of refs while iterating (https://github.com/libgit2/libgit2/pull/2395)

@carlosmn I wish I could marry you :heart::wink: