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/libostree-devel.sym

@@ -20,6 +20,7 @@
 /* Add new symbols here.  Release commits should copy this section into -released.sym. */
 
 LIBOSTREE_2017.15 {
+  ostree_repo_fsck_object;
 } LIBOSTREE_2017.14;
 
 /* Stub section for the stable release *after* this development one; don't