We have several pieces of information about subtrees: 1) What their current directory contents is 2) What their directory contents was at the time of the last subtree merge 3) What the directory contents of the upstream project is in the commit referred to by the subtree merge. Normally, all 3 should be identical. git-subtree-check.sh so far only compared (1) with (3) however. Fix this by comparing all three, and give some more useful diff output in the case of mismatch. The added benefit is that (1) and (2) can be compared without needing to see the upstream repository.
Backport of core PR11394