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

src/libostree/ostree-repo-private.h

@@ -92,6 +92,12 @@ struct OstreeRepo {
   OstreeRepo *parent_repo;
 };
 
+typedef struct {
+  dev_t dev;
+  ino_t ino;
+  char checksum[65];
+} OstreeDevIno;
+
 gboolean
 _ostree_repo_allocate_tmpdir (int           tmpdir_dfd,
                               const char   *tmpdir_prefix,