git-svn: fix several corner-case and rare bugs with 'commit'

git-svn: fix several corner-case and rare bugs with 'commit'

None of these were really show-stoppers (or even triggered)
on most of the trees I've tracked.

* Node change prevention for identically named nodes.  This is
  a limitation of SVN, but we find the error and exit before
  it's passed to SVN so we don't dirty our working tree when our
  commit fails.  git-svn will exit with an error code 1 if any
  of the following conditions are found:

  1.  a directory is removed and a file of the same name of the
      removed directory is created
  1a. a file has its parent directory removed and the file is
      takes the name of the removed parent directory::
          baz/zzz    =>  baz
  2.  a file is removed and a directory of the same name of the
      removed file is created.
  2a. a file is moved into a deeper directory that shares the
      previous name of the file::
          dir/$file  =>  dir/file/$file

  Since SVN cannot handle these cases, the user will have to
  manually split the commit into several parts.

* --rmdir now handles nested/deep removals. If dir/a/b/c/d/e/file
  is removed, and everything else is in the dir/ hierarchy is
  otherwise empty, then dir/ will be deleted when file is deleted
  from svn and --rmdir specified.

* Always assert that we have written the tree we want to write
  on commits.  This helped me find several bugs in the symlink
  handling code (which as been fixed).

* Several symlink handling fixes.  We now refuse to set
  permissions on symlinks.  We also always unlink a file
  if we're going to overwrite it.

* Apply changes in a pre-determined order, so we always have
  rename from locations handy before we delete them.

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>