am -3: list the paths that needed 3-way fallback

am -3: list the paths that needed 3-way fallback

When applying a patch that was based on an older release with "am -3", I
often wonder changes to which files need to be reviewed with extra care to
spot mismerges, but there is no good indication.

The paths that needed 3-way fallback can easily be obtained by comparing
the synthesized (partial) base tree and the current HEAD and noticing only
additions and modifications (removals only show the sparseness of the fake
ancestor tree, which is not useful information at all).  List them in the
usual --name-status format.

Signed-off-by: Junio C Hamano <gitster@pobox.com>