merge-recursive: Fix D/F conflicts

merge-recursive: Fix D/F conflicts

The D/F conflicts that can be automatically resolved (file or directory
unmodified on one side of history), have the nice property that
process_entry() can correctly handle all subpaths of the D/F conflict.  In
the case of D->F conversions, it will correctly delete all non-conflicting
files below the relevant directory and the directory itself (note that both
untracked and conflicting files below the directory will prevent its
removal).  So if we handle D/F conflicts after all other conflicts, they
become fairly simple to handle -- we just need to check for whether or not
a path (file/directory) is in the way of creating the new content.  We do
this by having process_entry() defer handling such entries to a subsequent
process_df_entry() step.

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