lib/repo: Add ostree_repo_equal() for comparing repos

This will compare their root directory inodes to see if they are the
same repository on disk. A convenience method for the users of the
public API who can’t access OstreeRepo.inode.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Closes: #1179
Approved by: cgwalters

src/libostree/libostree-devel.sym

@@ -19,6 +19,8 @@
 
 /* Add new symbols here.  Release commits should copy this section into -released.sym. */
 LIBOSTREE_2017.12 {
+global:
+  ostree_repo_equal;
 } LIBOSTREE_2017.11;