Make tests independent of global config files

This was done by setting $HOME to somewhere bogus. A better method is
to reuse $GIT_CONFIG, which was invented for ignoring the global
config file explicitely.

Technically, setting GIT_CONFIG=.git/config could be wrong, but it
passes all the tests, and we can keep the tests that way.

Signed-off-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>

t/test-lib.sh

@@ -255,8 +255,8 @@ test_done () {
 PATH=$(pwd)/..:$PATH
 GIT_EXEC_PATH=$(pwd)/..
 GIT_TEMPLATE_DIR=$(pwd)/../templates/blt
-HOME=$(pwd)/trash
-export PATH GIT_EXEC_PATH GIT_TEMPLATE_DIR HOME
+GIT_CONFIG=.git/config
+export PATH GIT_EXEC_PATH GIT_TEMPLATE_DIR GIT_CONFIG
 
 GITPERLLIB=$(pwd)/../perl/blib/lib:$(pwd)/../perl/blib/arch/auto/Git
 export GITPERLLIB