t0001: fix GIT_* environment variable check under --valgrind

When a test case is run without --valgrind, the wrap-for-bin.sh
helper script inserts the environment variable GIT_TEXTDOMAINDIR, but
when run with --valgrind, the variable is missing. A recently
introduced test case expects the presence of the variable, though, and
fails under --valgrind.

Rewrite the test case to strip conditially defined environment variables
from both expected and actual output.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>