test: Introduce $GIT_BUILD_DIR

Introduce a new variable $GIT_BUILD_DIR which can be used to locate
data that resides under the build directory, and use that instead.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/test-lib.sh

@@ -793,6 +793,7 @@ else # normal case, use ../bin-wrappers only unless $with_dashes:
 		PATH="$TEST_DIRECTORY/..:$PATH"
 	fi
 fi
+GIT_BUILD_DIR=$(pwd)/..
 GIT_TEMPLATE_DIR=$(pwd)/../templates/blt
 unset GIT_CONFIG
 GIT_CONFIG_NOSYSTEM=1