merge-recursive: Avoid working directory changes during recursive case

merge-recursive: Avoid working directory changes during recursive case

make_room_for_directories_of_df_conflicts() is about making sure necessary
working directory changes can succeed.  When o->call_depth > 0 (i.e. the
recursive case), we do not want to make any working directory changes so
this function should be skipped.

Note that make_room_for_directories_of_df_conflicts() is broken as has
been pointed out by Junio; it should NOT be unlinking files.  What it
should do is keep track of files that could be unlinked if a directory
later needs to be written in their place.  However, that work also is only
relevant in the non-recursive case, so this change is helpful either way.

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