git svn: attempt to create empty dirs on clone+rebase

git svn: attempt to create empty dirs on clone+rebase

We parse unhandled.log files for empty_dir statements and make a
best effort attempt to recreate empty directories on fresh
clones and rebase.  This should cover the majority of cases
where users work off a single branch or for projects where
branches do not differ in empty directories.

Since this cannot affect "normal" git commands like "checkout"
or "reset", so users switching between branches in a single
working directory should use the new "git svn mkdirs" command
after switching branches.

Signed-off-by: Eric Wong <normalperson@yhbt.net>