merge-recursive: do not check working copy when creating a virtual merge base

merge-recursive: do not check working copy when creating a virtual merge base

There were a few cases in merge-recursive that could result in a
check for the presence of files in the working copy while trying to
create a virtual merge base.  These were rare and innocuous, but
somewhat illogical.  The two cases were:

  * When there was naming conflicts (e.g. a D/F conflict) and we had to
    pick a new unique name for a file.  Since the new name is somewhat
    arbitrary, it didn't matter that we consulted the working copy to
    avoid picking a filename it has, but since the virtual merge base is
    never checked out, it's a waste of time and slightly odd to do so.

  * When two different files get renamed to the same name (on opposite
    sides of the merge), we needed to delete the original filenames from
    the cache and possibly also the working directory.  The caller's check
    for determining whether to delete from the working directory was a
    call to would_lose_untracked().  It turns out this didn't matter
    because remove_file() had logic to avoid modifying the working
    directory when creating a virtual merge base, but there is no reason
    for the caller to check the working directory in such circumstances.
    It's a waste of time, if not also a bit weird.

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