cherry-pick: Suggest a better method to retain authorship

cherry-pick: Suggest a better method to retain authorship

When a cherry-pick failed, we used to recommend setting environment
variables to retain the authorship. It is much easier, though, to use
the "-c" flag of git-commit.

Print this message also when merge-recursive fails (the code used to
exit(1) in that case, never reaching the proper failure path).

Signed-off-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>