rebase -i: Export GIT_AUTHOR_* variables explicitly

There is no point doing self-assignments of these variables. Instead, just export them to the environment, but do so in a sub-shell, because VAR1=VAL1 VAR2=VAL2 ... command arg1 arg2... does not mark the variables exported if command that is run is a shell function, according to POSIX.1. The callers of do_with_author do not rely on seeing the effect of any shell variable assignments that may happen inside what was called through this shell function (currently "output" is the only one), so running it in the subshell doesn't have an adverse semantic effect. Signed-off-by: Junio C Hamano <gitster@pobox.com>