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.c

@@ -2691,7 +2691,7 @@ min_free_space_size_validate_and_convert (OstreeRepo    *self,
         g_assert_not_reached ();
     }
 
-  self->min_free_space_size = g_ascii_strtoull (size_str, NULL, 10) << shifts;
+  self->min_free_space_mb = g_ascii_strtoull (size_str, NULL, 10) << shifts;
 
   return TRUE;
 }