checkout: add an extra checkout_overwrite mode

This is for issue projectatomic/rpm-ostree#365,
an extra option of overwrite mode is added to the checkout command
so that when there is "non-directory" file already exist
during checkout, the error will be handled.

Some tests are added for regression

Closes: #1116
Approved by: cgwalters

src/libostree/ostree-repo-checkout.c

@@ -654,7 +654,8 @@ checkout_tree_at_recurse (OstreeRepo                        *self,
       {
         if (errno == EEXIST &&
             (options->overwrite_mode == OSTREE_REPO_CHECKOUT_OVERWRITE_UNION_FILES
-             || options->overwrite_mode == OSTREE_REPO_CHECKOUT_OVERWRITE_ADD_FILES))
+             || options->overwrite_mode == OSTREE_REPO_CHECKOUT_OVERWRITE_ADD_FILES
+             || options->overwrite_mode == OSTREE_REPO_CHECKOUT_OVERWRITE_DISJOINT_UNION_FILES))
           did_exist = TRUE;
         else
           return glnx_throw_errno (error);