tests/testlib/common.sh

manifest: improve error message in case for tree manifest

In the case where tree manifest is not enabled but we still receive an sub
directory information for the manifest. The error now inform which sub-directory
was passed.

mkcommit() {
   name="$1"
   shift
   echo "$name" > "$name"
   hg add "$name"
   hg ci -m "$name" "$@"
}