lib/repo: Add a DEVINO_CANONICAL commit modifier flag

I was seeing the `Writing OSTree commit...` phase of rpm-ostree
being very slow lately.  This turns out to be more fallout from
https://github.com/ostreedev/ostree/pull/1170
AKA commit: 8fe4536

Loading the xattrs is slow on my system (F27AW, XFS+LVM, NVMe). I haven't fully
traced through why, but AIUI at least on XFS the xattrs are often stored outside
of the inode so it's a little bit like doing an `open()+read()`. Plus there's
the LSM overhead, etc.

The thing is that for rpm-ostree's package layering use case, we
basically always want to treat the on-disk state as canonical.  (There's
a subtle case here if one does overrides for something that contains
policy but we'll fix that).

Anyways, so we're in a state now where we do the slow but correct thing by
default, which seems sane. But let's allow the app to opt-in to telling us
"really trust devino". The difference between a `stat()` + hash table lookup
versus the full xattr load on my test case of `rpm-ostree install
./tree-1.7.0-10.fc27.x86_64.rpm` is absolutely dramatic; consistently on the
order of 10s without this support, and <1s with (800ms).

Closes: #1357
Approved by: jlebon

src/libostree/ostree-repo.h

@@ -644,6 +644,7 @@ typedef OstreeRepoCommitFilterResult (*OstreeRepoCommitFilter) (OstreeRepo    *r
  * @OSTREE_REPO_COMMIT_MODIFIER_FLAGS_CANONICAL_PERMISSIONS: Canonicalize permissions for bare-user-only mode.
  * @OSTREE_REPO_COMMIT_MODIFIER_FLAGS_ERROR_ON_UNLABELED: Emit an error if configured SELinux policy does not provide a label
  * @OSTREE_REPO_COMMIT_MODIFIER_FLAGS_CONSUME: Delete added files/directories after commit; Since: 2017.13
+ * @OSTREE_REPO_COMMIT_MODIFIER_FLAGS_DEVINO_CANONICAL: If a devino cache hit is found, skip modifier filters (non-directories only); Since: 2017.14
  */
 typedef enum {
   OSTREE_REPO_COMMIT_MODIFIER_FLAGS_NONE = 0,
@@ -652,6 +653,7 @@ typedef enum {
   OSTREE_REPO_COMMIT_MODIFIER_FLAGS_CANONICAL_PERMISSIONS = (1 << 2),
   OSTREE_REPO_COMMIT_MODIFIER_FLAGS_ERROR_ON_UNLABELED = (1 << 3),
   OSTREE_REPO_COMMIT_MODIFIER_FLAGS_CONSUME = (1 << 4),
+  OSTREE_REPO_COMMIT_MODIFIER_FLAGS_DEVINO_CANONICAL = (1 << 5),
 } OstreeRepoCommitModifierFlags;
 
 /**