lib/commit: disallow writing content in 'bare-split-xattrs' mode

This prevents writing content into 'bare-split-xattrs` repository,
while carving some space for experimenting via a temporary
`OSTREE_EXP_WRITE_BARE_SPLIT_XATTRS` environment flag.

tests/test-basic-bare-split-xattrs.sh

@@ -22,4 +22,26 @@ ${OSTREE} fsck --all
 tap_ok "repo fsck"
 rm -rf -- repo
 
+cd ${test_tmpdir}
+mkdir -p "${test_tmpdir}/files"
+touch files/foo
+${OSTREE} init --mode "${mode}"
+if ${OSTREE} commit --orphan -m "not implemented" files; then
+    assert_not_reached "commit to bare-split-xattrs should have failed"
+fi
+${OSTREE} fsck --all
+tap_ok "commit not implemented"
+rm -rf -- repo files
+
+cd ${test_tmpdir}
+mkdir -p "${test_tmpdir}/files"
+touch files/foo
+${OSTREE} init --mode "${mode}"
+OSTREE_EXP_WRITE_BARE_SPLIT_XATTRS=true ${OSTREE} commit --orphan -m "experimental" files
+if ${OSTREE} fsck --all; then
+    assert_not_reached "fsck should have failed"
+fi
+tap_ok "commit exp override"
+rm -rf -- repo files
+
 tap_end