lib/repo: Separate min-free-space-* calculation from transaction codepath

lib/repo: Separate min-free-space-* calculation from transaction codepath

Earlier, the actual reserved space (in blocks) were calculated inside the
transaction codepath ostree_repo_prepare_transaction(). However, while
reworking on ostree_repo_get_min_free_space_bytes() API, it was realized that
this calculation can be done independently from the transaction's codepaths, hence
enabling the usage for ostree_repo_get_min_free_space_bytes() API irrespective
of whether there is an ongoing transaction or not.

https://github.com/ostreedev/ostree/issues/1720

Closes: #1722
Approved by: pwithnall