repo: Add APIs for devino optimization between checkout -> commit

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

doc/ostree-sections.txt

@@ -276,8 +276,12 @@ ostree_repo_commit_modifier_new
 OstreeRepoCommitModifierXattrCallback
 ostree_repo_commit_modifier_set_xattr_callback
 ostree_repo_commit_modifier_set_sepolicy
+ostree_repo_commit_modifier_set_devino_cache
 ostree_repo_commit_modifier_ref
 ostree_repo_commit_modifier_unref
+ostree_repo_devino_cache_new
+ostree_repo_devino_cache_ref
+ostree_repo_devino_cache_unref
 ostree_repo_write_directory_to_mtree
 ostree_repo_write_dfd_to_mtree
 ostree_repo_write_archive_to_mtree