git-svn: add an option to skip the creation of empty directories

git-svn: add an option to skip the creation of empty directories

"git svn mkdirs" (which creates empty directories in the current
working copy) can be very slow and is often unnecessary.  Provide a
config file option "svn-remote.<name>.automkdirs" that prevents empty
directories from being created automatically.  (They are still created
if "git svn mkdirs" is invoked explicitly.)

Based-on-patch-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>