make git barf when an alias changes environment variables

make git barf when an alias changes environment variables

Aliases changing environment variables (GIT_DIR or
GIT_WORK_TREE) can cause problems:
git has to use GIT_DIR to read the aliases from the config.
After running handle_options for the alias the options of the
alias may have changed environment variables.  Depending on
the implementation of setenv the memory location obtained
through getenv earlier may contain the old value or the new
value (or even be used for something else?).  To avoid these
problems git errors out if an alias uses any option which
changes environment variables.

Signed-off-by: Matthias Lederhofer <matled@gmx.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>