t2016: require the PERL prereq only when necessary

The scripted version of the interactive mode of `git add` still requires
Perl, but the built-in version does not. Let's only require the PERL
prereq if testing the scripted version.

This addresses a long-standing NEEDSWORK added in 35166b1 (t2016:
add a NEEDSWORK about the PERL prerequisite, 2020-10-07).

Signed-off-by: Johannes Schindelin <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: dscho

t/t2016-checkout-patch.sh

@@ -4,7 +4,13 @@ test_description='git checkout --patch'
 
 . ./lib-patch-mode.sh
 
-test_expect_success PERL 'setup' '
+if ! test_bool_env GIT_TEST_ADD_I_USE_BUILTIN false && ! test_have_prereq PERL
+then
+	skip_all='skipping interactive add tests, PERL not set'
+	test_done
+fi
+
+test_expect_success 'setup' '
 	mkdir dir &&
 	echo parent > dir/foo &&
 	echo dummy > bar &&
@@ -18,66 +24,62 @@ test_expect_success PERL 'setup' '
 
 # note: bar sorts before dir/foo, so the first 'n' is always to skip 'bar'
 
-# NEEDSWORK: Since the builtin add-p is used when $GIT_TEST_ADD_I_USE_BUILTIN
-# is given, we should replace the PERL prerequisite with an ADD_I prerequisite
-# which first checks if $GIT_TEST_ADD_I_USE_BUILTIN is defined before checking
-# PERL.
-test_expect_success PERL 'saying "n" does nothing' '
+test_expect_success 'saying "n" does nothing' '
 	set_and_save_state dir/foo work head &&
 	test_write_lines n n | git checkout -p &&
 	verify_saved_state bar &&
 	verify_saved_state dir/foo
 '
 
-test_expect_success PERL 'git checkout -p' '
+test_expect_success 'git checkout -p' '
 	test_write_lines n y | git checkout -p &&
 	verify_saved_state bar &&
 	verify_state dir/foo head head
 '
 
-test_expect_success PERL 'git checkout -p with staged changes' '
+test_expect_success 'git checkout -p with staged changes' '
 	set_state dir/foo work index &&
 	test_write_lines n y | git checkout -p &&
 	verify_saved_state bar &&
 	verify_state dir/foo index index
 '
 
-test_expect_success PERL 'git checkout -p HEAD with NO staged changes: abort' '
+test_expect_success 'git checkout -p HEAD with NO staged changes: abort' '
 	set_and_save_state dir/foo work head &&
 	test_write_lines n y n | git checkout -p HEAD &&
 	verify_saved_state bar &&
 	verify_saved_state dir/foo
 '
 
-test_expect_success PERL 'git checkout -p HEAD with NO staged changes: apply' '
+test_expect_success 'git checkout -p HEAD with NO staged changes: apply' '
 	test_write_lines n y y | git checkout -p HEAD &&
 	verify_saved_state bar &&
 	verify_state dir/foo head head
 '
 
-test_expect_success PERL 'git checkout -p HEAD with change already staged' '
+test_expect_success 'git checkout -p HEAD with change already staged' '
 	set_state dir/foo index index &&
 	# the third n is to get out in case it mistakenly does not apply
 	test_write_lines n y n | git checkout -p HEAD &&
 	verify_saved_state bar &&
 	verify_state dir/foo head head
 '
 
-test_expect_success PERL 'git checkout -p HEAD^...' '
+test_expect_success 'git checkout -p HEAD^...' '
 	# the third n is to get out in case it mistakenly does not apply
 	test_write_lines n y n | git checkout -p HEAD^... &&
 	verify_saved_state bar &&
 	verify_state dir/foo parent parent
 '
 
-test_expect_success PERL 'git checkout -p HEAD^' '
+test_expect_success 'git checkout -p HEAD^' '
 	# the third n is to get out in case it mistakenly does not apply
 	test_write_lines n y n | git checkout -p HEAD^ &&
 	verify_saved_state bar &&
 	verify_state dir/foo parent parent
 '
 
-test_expect_success PERL 'git checkout -p handles deletion' '
+test_expect_success 'git checkout -p handles deletion' '
 	set_state dir/foo work index &&
 	rm dir/foo &&
 	test_write_lines n y | git checkout -p &&
@@ -90,40 +92,40 @@ test_expect_success PERL 'git checkout -p handles deletion' '
 # dir/foo.  There's always an extra 'n' to reject edits to dir/foo in
 # the failure case (and thus get out of the loop).
 
-test_expect_success PERL 'path limiting works: dir' '
+test_expect_success 'path limiting works: dir' '
 	set_state dir/foo work head &&
 	test_write_lines y n | git checkout -p dir &&
 	verify_saved_state bar &&
 	verify_state dir/foo head head
 '
 
-test_expect_success PERL 'path limiting works: -- dir' '
+test_expect_success 'path limiting works: -- dir' '
 	set_state dir/foo work head &&
 	test_write_lines y n | git checkout -p -- dir &&
 	verify_saved_state bar &&
 	verify_state dir/foo head head
 '
 
-test_expect_success PERL 'path limiting works: HEAD^ -- dir' '
+test_expect_success 'path limiting works: HEAD^ -- dir' '
 	# the third n is to get out in case it mistakenly does not apply
 	test_write_lines y n n | git checkout -p HEAD^ -- dir &&
 	verify_saved_state bar &&
 	verify_state dir/foo parent parent
 '
 
-test_expect_success PERL 'path limiting works: foo inside dir' '
+test_expect_success 'path limiting works: foo inside dir' '
 	set_state dir/foo work head &&
 	# the third n is to get out in case it mistakenly does not apply
 	test_write_lines y n n | (cd dir && git checkout -p foo) &&
 	verify_saved_state bar &&
 	verify_state dir/foo head head
 '
 
-test_expect_success PERL 'none of this moved HEAD' '
+test_expect_success 'none of this moved HEAD' '
 	verify_saved_head
 '
 
-test_expect_success PERL 'empty tree can be handled' '
+test_expect_success 'empty tree can be handled' '
 	test_when_finished "git reset --hard" &&
 	git checkout -p $(test_oid empty_tree) --
 '