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

apidoc/ostree-sections.txt

@@ -379,6 +379,7 @@ ostree_repo_commit_modifier_new
 OstreeRepoCommitModifierXattrCallback
 ostree_repo_commit_modifier_set_xattr_callback
 ostree_repo_commit_modifier_set_sepolicy
+ostree_repo_commit_modifier_set_sepolicy_from_commit
 ostree_repo_commit_modifier_set_devino_cache
 ostree_repo_commit_modifier_ref
 ostree_repo_commit_modifier_unref