fast-import: Improve robustness when D->F changes provided in wrong order

fast-import: Improve robustness when D->F changes provided in wrong order

When older versions of fast-export came across a directory changing to a
symlink (or regular file), it would output the changes in the form
  M 120000 :239821 dir-changing-to-symlink
  D dir-changing-to-symlink/filename1
When fast-import sees the first line, it deletes the directory named
dir-changing-to-symlink (and any files below it) and creates a symlink in
its place.  When fast-import came across the second line, it was previously
trying to remove the file and relevant leading directories in
tree_content_remove(), and as a side effect it would delete the symlink
that was just created.  This resulted in the symlink silently missing from
the resulting repository.

To improve robustness, we ignore file deletions underneath directory names
that correspond to non-directories.  This can also be viewed as a minor
optimization: since there cannot be a file and a directory with the same
name in the same directory, the file clearly can't exist so nothing needs
to be done to delete it.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>