* Rework GIT_DIRREMOVAL values to GIT_RMDIR flags, allowing
  combinations of flags
* Add GIT_RMDIR_EMPTY_PARENTS flag to remove parent dirs that
  are left empty after removal
* Add GIT_MKDIR_VERIFY_DIR to give an error if item is a file,
  not a dir (previously an EEXISTS error was ignored, even for
  files) and enable this flag for git_futils_mkpath2file call
* Improve accuracy of error messages from git_futils_mkdir

This started as a complex new test for checkout going through the
"typechanges" test repository, but that revealed numerous issues
with checkout, including:

* complete failure with submodules
* failure to create blobs with exec bits
* problems when replacing a tree with a blob because the tree
  "example/" sorts after the blob "example" so the delete was
  being processed after the single file blob was created

This fixes most of those problems and includes a number of other
minor changes that made it easier to do that, including improving
the TYPECHANGE support in diff/status, etc.

repository: ensure git_repository_discover() returns ENOTFOUND when unable to find a repository given the constraints

It'd be nice to be rid of all the #define's, but converting to const
char*'s would be even messier, and less declarative.