lib/repo: Rename free_space_size variable to free_space_mb

I generally like having variables include their units where applicable;
timer variables having `_secs` or `_ms`, etc.

Closes: #1632
Approved by: jlebon

src/libostree/ostree-repo-private.h

@@ -151,7 +151,7 @@ struct OstreeRepo {
   uid_t target_owner_uid; /* Ensure files are chowned to this uid/gid */
   gid_t target_owner_gid;
   guint min_free_space_percent; /* See the min-free-space-percent config option */
-  guint64 min_free_space_size; /* See the min-free-space-size config option */
+  guint64 min_free_space_mb; /* See the min-free-space-size config option */
 
   guint test_error_flags; /* OstreeRepoTestErrorFlags */