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-types.h

@@ -27,6 +27,7 @@
 G_BEGIN_DECLS
 
 typedef struct OstreeRepo OstreeRepo;
+typedef struct OstreeRepoDevInoCache OstreeRepoDevInoCache;
 typedef struct OstreeSePolicy OstreeSePolicy;
 typedef struct OstreeSysroot OstreeSysroot;
 typedef struct OstreeSysrootUpgrader OstreeSysrootUpgrader;