lib/repo: Add an API to mark a commit as partial

For the [rpm-ostree jigdo ♲📦](https://github.com/projectatomic/rpm-ostree/issues/1081) work.
We're basically doing "pull" via a non-libostree mechanism, and this
should be fully supported.  As I mentioned earlier we should try to
have `ostree-repo-pull.c` only use public APIs; this gets us closer
to that.

Closes: #1376
Approved by: jlebon

src/libostree/libostree-devel.sym

@@ -21,6 +21,7 @@
 
 LIBOSTREE_2017.15 {
   ostree_repo_fsck_object;
+  ostree_repo_mark_commit_partial;
 } LIBOSTREE_2017.14;
 
 /* Stub section for the stable release *after* this development one; don't