wrap-for-bin: make bin-wrappers chainable

wrap-for-bin: make bin-wrappers chainable

For now, bin-wrappers overwrites GITPERLLIB. If we want to chain to
those scripts and define GITPERLLIB before, our changes will be
discarded.

This patch makes the bin-wrappers prepend their modifications to
GITPERLLIB rather than redefining it. It also unset GITPERLLIB in the
test-suite to prevent broken $GITPERLLIB in the user's configuration
from interfering with the testsuite.

The codes using GIT_TEMPLATE_DIR and GIT_TEXTDOMAINDIR handle only one
path in each of this variable so this new behavior would be useless on
those variables.

Signed-off-by: Benoit Person <benoit.person@ensimag.fr>
Signed-off-by: Matthieu Moy <matthieu.moy@grenoble-inp.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>