submodule: Demonstrate known breakage during recursive merge

submodule: Demonstrate known breakage during recursive merge

Since commit 68d03e4a (Implement automatic fast-forward merge for
submodules, 2010-07-07) we try to suggest submodule commits that resolve
a conflict.  Consider a true recursive merge case

    b---bc
   / \ /
  o   X
   \ / \
    c---cb

in which the two heads themselves (bc,cb) had resolved a submodule
conflict (i.e. reference different commits than their parents).  The
submodule merge search runs during the temporary merge of the two merge
bases (b,c) and prints out a suggestion that is not meaningful to the
user.  Then during the main merge the submodule merge search runs again
but dies with the message

  fatal: --ancestry-path given but there are no bottom commits

while trying to enumerate candidates.  Demonstrate this known breakage
with a new test in t7405-submodule-merge covering the case.

Signed-off-by: Brad King <brad.king@kitware.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>