am: stop exporting GIT_COMMITTER_DATE

am: stop exporting GIT_COMMITTER_DATE

The implementation of --committer-date-is-author-date exports
GIT_COMMITTER_DATE to override the default committer date but does not
reset GIT_COMMITTER_DATE in the environment after creating the commit
so it is set in the environment of any hooks that get run. We're about
to add the same functionality to the sequencer and do not want to have
GIT_COMMITTER_DATE set when running hooks or exec commands so lets
update commit_tree_extended() to take an explicit committer so we
override the default date without setting GIT_COMMITTER_DATE in the
environment.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>