repo/commit: Add support for --selinux-policy-from-base

The [dev-overlay](https://github.com/coreos/coreos-assembler/blob/332c6ab3b91778d904224c3c960d9cc4739d60bd/src/cmd-dev-overlay)
script shipped in coreos-assembler mostly exists to deal
with the nontrivial logic around SELinux policy.  Let's make
the use case of "commit some binaries overlaying a base tree, using
the base's selinux policy" just require a magical
`--selinux-policy-from-base` argument to `ostree commit`.

A new C API was added to implement this in the case of `--tree=ref`;
when the base directory is already checked out, we can just reuse
the existing logic that `--selinux-policy` was using.

Requires: https://github.com/ostreedev/ostree/pull/2039

src/libostree/libostree-devel.sym

@@ -20,7 +20,7 @@
 /* Add new symbols here.  Release commits should copy this section into -released.sym. */
 LIBOSTREE_2020.2 {
 global:
-  someostree_symbol_deleteme;
+  ostree_repo_commit_modifier_set_sepolicy_from_commit;
 } LIBOSTREE_2020.1;
 
 /* Stub section for the stable release *after* this development one; don't