bin/delta: Fix compilation with relative subdirs --filename

Currently we were parsing `opt_filename` twice...I dug through
the history a bit and it looks like it may have been an accident
from refactoring.

What we're fixing here concretely is that using relative subdirectories
like `--filename somesubdir/foo` broke because we were incorrectly
passing the `somesubdir/` again.

Closes: #1423

Closes: #1427
Approved by: jlebon

tests/test-delta.sh

@@ -123,8 +123,9 @@ fi
 
 ostree_repo_init temp-repo --mode=archive
 ${CMD_PREFIX} ostree --repo=temp-repo pull-local repo
-${CMD_PREFIX} ostree --repo=temp-repo static-delta generate --empty --to=${newrev} --filename=some.delta
-assert_has_file some.delta
+mkdir tmpsubdir
+${CMD_PREFIX} ostree --repo=temp-repo static-delta generate --empty --to=${newrev} --filename=tmpsubdir/some.delta
+assert_has_file tmpsubdir/some.delta
 ${CMD_PREFIX} ostree --repo=temp-repo static-delta list > delta-list.txt
 assert_file_has_content delta-list.txt 'No static deltas'
 rm temp-repo -rf