Merge branch 'as/subtree-with-spaces' into maint

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