tests: unset COLUMNS inherited from environment

$COLUMNS must be unset to not interfere with the tests. The tests
already ignore the terminal size because output is redirected to a
file, but COLUMNS overrides terminal size detection and changes the
test output away from the standard 80.
Reported-by: NJeff King <peff@peff.net>
Signed-off-by: NZbigniew Jędrzejewski-Szmek <zbyszek@in.waw.pl>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

t/test-lib.sh

@@ -46,7 +46,7 @@ EDITOR=:
 # /usr/xpg4/bin/sh and /bin/ksh to bail out.  So keep the unsets
 # deriving from the command substitution clustered with the other
 # ones.
-unset VISUAL EMAIL LANGUAGE $(perl -e '
+unset VISUAL EMAIL LANGUAGE COLUMNS $(perl -e '
 	my @env = keys %ENV;
 	my $ok = join("|", qw(
 		TRACE