Merge branch 'as/subtree-with-spaces'

Update "git subtree" (in contrib/) so that it can take whitespaces
in the pathnames, not only in the in-tree pathname but the name of
the directory that the repository is in.

* as/subtree-with-spaces:
  contrib/subtree: respect spaces in a repository path
  t7900-subtree: test the "space in a subdirectory name" case