Print conflict details with the new infrastructure (bug #11181)

The patch modifies the IndexAndWorkTree.merge() function to display
pass the conflict information (files) when raising an exception. The
logic is similar to the one in the old infrastructure.

Signed-off-by: Catalin Marinas <catalin.marinas@gmail.com>
Acked-by: Karl Hasselström <kha@treskal.com>