bsdtar's mtree format has a quirk wherein entries without "/" in their
first word are treated as "relative" entries, and "relative" directories
will cause tar to "change directory" into the declared directory entry.
If such a directory is followed by a "relative" entry, then the file
will be created within the directory, instead of at top-level as
expected. To mitigate, we append a slash to top-level directory entries.

Fixes bazel-contrib#851.