apply: get rid of --index-info in favor of --build-fake-ancestor

apply: get rid of --index-info in favor of --build-fake-ancestor

git-am used "git apply -z --index-info" to find the original versions
of the files touched by the diff, to be able to do an inexpensive
three-way merge.

This operation makes only sense in a repository, since the index
information in the diff refers to blobs, which have to be present in
the current repository.

Therefore, teach "git apply" a mode to write out the result as an
index file to begin with, obviating the need for scripts to do it
themselves.

The sole user for --index-info is "git am" is converted to
use --build-fake-ancestor in this patch.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>