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.h

@@ -418,6 +418,9 @@ void ostree_repo_commit_modifier_set_xattr_callback (OstreeRepoCommitModifier
 void ostree_repo_commit_modifier_set_sepolicy (OstreeRepoCommitModifier              *modifier,
                                                OstreeSePolicy                        *sepolicy);
 
+void ostree_repo_commit_modifier_set_devino_cache (OstreeRepoCommitModifier              *modifier,
+                                                   OstreeRepoDevInoCache                 *cache);
+
 OstreeRepoCommitModifier *ostree_repo_commit_modifier_ref (OstreeRepoCommitModifier *modifier);
 void ostree_repo_commit_modifier_unref (OstreeRepoCommitModifier *modifier);
 
@@ -531,10 +534,17 @@ typedef struct {
 
   const char *subpath;
 
+  OstreeRepoDevInoCache *devino_to_csum_cache;
+
   guint unused_uints[6];
-  gpointer unused_ptrs[8];
+  gpointer unused_ptrs[7];
 } OstreeRepoCheckoutOptions;
 
+GType ostree_repo_devino_cache_get_type (void);
+OstreeRepoDevInoCache *ostree_repo_devino_cache_new (void);
+OstreeRepoDevInoCache * ostree_repo_devino_cache_ref (OstreeRepoDevInoCache *cache);
+void ostree_repo_devino_cache_unref (OstreeRepoDevInoCache *cache);
+
 gboolean ostree_repo_checkout_tree_at (OstreeRepo                         *self,
                                        OstreeRepoCheckoutOptions          *options,
                                        int                                 destination_dfd,