builtin/apply: move 'fake_ancestor' global into 'struct apply_state'

builtin/apply: move 'fake_ancestor' global into 'struct apply_state'

To libify the apply functionality the 'fake_ancestor' variable should
not be static and global to the file. Let's move it into
'struct apply_state'.

By the way remove a comment about '--index-info' that was renamed
'--build-fake-ancestor' in commit 26b28007689d27a921ea90e5a29fc8eb74b0d297
(apply: get rid of --index-info in favor of --build-fake-ancestor,
Sep 17 2007).

Reviewed-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>