Add public API for fsck, use it before loading metadata

A while ago I did `truncate -s 0 /path/to/repo/00/123.commit`, and expected a
checksum error, but I actually got a validation error due to us loading the
commit into a variant and trying to parse out the parent checksum, etc.

I first started by changing the `load_and_fsck_one_object()` function to
checksum before loading, but the problem is that we do a traverse of all objects
first. Fixing this is going to require an `OSTREE_REPO_COMMIT_TRAVER_FLAG_FSCK`
or something.

In the meantime at least though, let's add a public API to fsck a single object
which *does* checksum cleanly before parsing the object, and change the `fsck`
command to use it.

We then change the fsck binary to do this while iterating over the refs
and finding the commit object.  This way we'll at least get a checksum
first for commit objects, even if not dirtree/dirmeta.

Closes: #1364
Approved by: jlebon

src/libostree/ostree-repo.h

@@ -650,10 +650,17 @@ gboolean      ostree_repo_import_object_from_with_trust (OstreeRepo           *s
 _OSTREE_PUBLIC
 gboolean      ostree_repo_delete_object (OstreeRepo           *self,
                                          OstreeObjectType      objtype,
-                                         const char           *sha256, 
+                                         const char           *sha256,
                                          GCancellable         *cancellable,
                                          GError              **error);
 
+_OSTREE_PUBLIC
+gboolean      ostree_repo_fsck_object (OstreeRepo           *self,
+                                       OstreeObjectType      objtype,
+                                       const char           *sha256,
+                                       GCancellable         *cancellable,
+                                       GError              **error);
+
 /** 
  * OstreeRepoCommitFilterResult:
  * @OSTREE_REPO_COMMIT_FILTER_ALLOW: Do commit this object