A fast way to generate new OSTree content using an existing
tree is to checkout (as hard links), add/replace files, then
call `ostree_repo_scan_hardlinks()`, then commit.
But `ostree_repo_scan_hardlinks()` scans the entire repo, which
can be slow if you have a lot of content.
All we really need is a mapping of (device,inode) -> checksum
just for the objects we checked out, then use that mapping
for commits.
This patch adds API so that callers can create a mapping via
`ostree_repo_devino_cache_new()`, then pass it to
`ostree_repo_checkout_tree_at()` which will populate it, and then
`ostree_repo_write_directory_to_mtree()` can consume it.
I plan to use this in rpm-ostree for package layering work.
Notes:
- The old `ostree_repo_scan_hardlinks()` API still works.
- I tweaked the cache to be a set with the checksum colocated with
the key, to avoid a separate malloc block per entry.
https://github.com/GNOME/ostree/pull/167
5929ce9e0e