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>

t/test-lib.sh

@@ -92,6 +92,7 @@ unset VISUAL EMAIL LANGUAGE COLUMNS $("$PERL_PATH" -e '
 	print join("\n", @vars);
 ')
 unset XDG_CONFIG_HOME
+unset GITPERLLIB
 GIT_AUTHOR_EMAIL=author@example.com
 GIT_AUTHOR_NAME='A U Thor'
 GIT_COMMITTER_EMAIL=committer@example.com

wrap-for-bin.sh

@@ -14,7 +14,7 @@ else
 	GIT_TEMPLATE_DIR='@@BUILD_DIR@@/templates/blt'
 	export GIT_TEMPLATE_DIR
 fi
-GITPERLLIB='@@BUILD_DIR@@/perl/blib/lib'
+GITPERLLIB='@@BUILD_DIR@@/perl/blib/lib'"${GITPERLLIB:+:$GITPERLLIB}"
 GIT_TEXTDOMAINDIR='@@BUILD_DIR@@/po/build/locale'
 PATH='@@BUILD_DIR@@/bin-wrappers:'"$PATH"
 export GIT_EXEC_PATH GITPERLLIB PATH GIT_TEXTDOMAINDIR