bin/refs: Allow overwriting existing ref

Currently if you want to update a non-alias ref, you need to first check
if it exists and use either `ostree refs --create` or `ostree reset` as
appropriate. That's unnecessarily complicated and is much less
convenient than the old `write-refs` builtin that simply called
`ostree_repo_set_ref_immediate()` without any checks.

Add a `--force` option to be used with `--create` that does not raise an
error when the destination ref already exists.

Closes: #1870
Approved by: jlebon

tests/test-refs.sh

@@ -90,6 +90,11 @@ if ${CMD_PREFIX} ostree --repo=repo refs foo/ctest --create=ctest; then
     assert_not_reached "refs --create unexpectedly succeeded in overwriting an existing prefix!"
 fi
 
+# Force overwriting ctest and check the revision got updated
+foo_ctest_rev=$(${CMD_PREFIX} ostree --repo=repo rev-parse foo/ctest)
+${CMD_PREFIX} ostree --repo=repo refs foo/ctest --create=ctest --force
+assert_ref repo ctest ${foo_ctest_rev}
+
 # https://github.com/ostreedev/ostree/issues/1285
 # One tool was creating .latest_rsync files in each dir, let's ignore stuff like
 # that.