t5608-clone-2gb.sh: turn GIT_TEST_CLONE_2GB into a bool

The GIT_TEST_CLONE_2GB environment variable is only ever checked with
'test -z' in 't5608-clone-2gb.sh', so any non-empty value is
interpreted as "yes, run these expensive tests", even
'GIT_TEST_CLONE_2GB=NoThanks'.

Similar GIT_TEST_* environment variables have already been turned into
bools in 3b072c577b (tests: replace test_tristate with "git
env--helper", 2019-06-21), so let's turn GIT_TEST_CLONE_2GB into a
bool as well, to follow suit.

Our CI builds set GIT_TEST_CLONE_2GB=YesPlease, so adjust them
accordingly, thus removing the last 'YesPlease' from our CI scripts.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

ci/lib.sh

@@ -156,7 +156,7 @@ fi
 
 export DEVELOPER=1
 export DEFAULT_TEST_TARGET=prove
-export GIT_TEST_CLONE_2GB=YesPlease
+export GIT_TEST_CLONE_2GB=true
 
 case "$jobname" in
 linux-clang|linux-gcc)

t/t5608-clone-2gb.sh

@@ -3,7 +3,7 @@
 test_description='Test cloning a repository larger than 2 gigabyte'
 . ./test-lib.sh
 
-if test -z "$GIT_TEST_CLONE_2GB"
+if ! test_bool_env GIT_TEST_CLONE_2GB false
 then
 	say 'Skipping expensive 2GB clone test; enable it with GIT_TEST_CLONE_2GB=t'
 else