merge-recursive: Make room for directories in D/F conflicts

merge-recursive: Make room for directories in D/F conflicts

When there are unmerged entries present, make sure to check for D/F
conflicts first and remove any files present in HEAD that would be in the
way of creating files below the correspondingly named directory.  Such
files will be processed again at the end of the merge in
process_df_entry(); at that time we will be able to tell if we need to
and can reinstate the file, whether we need to place its contents in a
different file due to the directory still being present, etc.

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