commit: Add _CONSUME modifier flag

For many cases of commit, we can actually optimize things by simply "adopting"
the object rather than writing a new copy. For example, in rpm-ostree package
layering.

We can only make that optimization though if we take ownership of the file. This
commit hence adds an API where a caller tells us to do so. For now, that just
means we `unlink()` the files/dirs as we go, but we can now later add the
"adopt" optimization.

Closes: #1255
Approved by: jlebon

src/libostree/ostree-repo.h

@@ -630,6 +630,7 @@ typedef OstreeRepoCommitFilterResult (*OstreeRepoCommitFilter) (OstreeRepo    *r
  * @OSTREE_REPO_COMMIT_MODIFIER_FLAGS_GENERATE_SIZES: Generate size information.
  * @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
  */
 typedef enum {
   OSTREE_REPO_COMMIT_MODIFIER_FLAGS_NONE = 0,
@@ -637,6 +638,7 @@ typedef enum {
   OSTREE_REPO_COMMIT_MODIFIER_FLAGS_GENERATE_SIZES = (1 << 1),
   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),
 } OstreeRepoCommitModifierFlags;
 
 /**