archive: Use an empty directory for submodules

archive: Use an empty directory for submodules

When encountering a submodule entry, "jgit archive" tries to write its
content verbatim as an entry to the archive, which fails with a
JGitInternalException wrapping a MissingObjectException because the
submodule repository commits are typically not part of the
superproject.

When a subproject is available (for example because it has been
checked out as a subdirectory of a superproject worktree), it would be
nice to recurse into it and make one archive recording the state of
the entire project.  Unfortunately sometimes the subproject is not
available or it can be hard to find (e.g., it can be on another
server).  Even when some subprojects are available, "jgit archive"
should not produce different output for the same tree depending on
which subprojects it has easy access to, so there is no obvious good
default behavior that recurses without relying on access to all
subprojects.

Instead, replace each submodule entry with a placeholder empty
directory.  "git archive" does the same.

Change-Id: I1295086037b77fc948b3f93c21d47341e25483e5
Signed-off-by: Jonathan Nieder <jrn@google.com>