checkout: don't require a work tree when checking out into a new one

For normal use cases, it does not make sense for 'checkout' to work on
a bare repository, without a worktree. But "checkout --to" is an
exception because it _creates_ a new worktree. Allow this option to
run on bare repositories.

People who check out from a bare repository should remember that
core.logallrefupdates is off by default and it should be turned back
on. `--to` cannot do this automatically behind the user's back because
some user may deliberately want no reflog.

For people interested in repository setup/discovery code,
is_bare_repository_cfg (aka "core.bare") is unchanged by this patch,
which means 'true' by default for bare repos. Fortunately when we get
the repo through a linked checkout, is_bare_repository_cfg is never
used. So all is still good.

[nd: commit message]

Signed-off-by: Dennis Kaarsemaker <dennis@kaarsemaker.net>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/checkout.c

diff --git a/builtin/checkout.c b/builtin/checkout.c
index 5dfdbda..8c5276c 100644
--- a/builtin/checkout.c
+++ b/builtin/checkout.c
@@ -1364,6 +1364,9 @@
 	if (opts.new_worktree_mode)
 		opts.new_worktree = NULL;
 
+	if (!opts.new_worktree)
+		setup_work_tree();
+
 	if (conflict_style) {
 		opts.merge = 1; /* implied */
 		git_xmerge_config("merge.conflictstyle", conflict_style, NULL);

git.c

diff --git a/git.c b/git.c
index 18fbf79..160896a 100644
--- a/git.c
+++ b/git.c
@@ -383,7 +383,7 @@
 	{ "check-ignore", cmd_check_ignore, RUN_SETUP | NEED_WORK_TREE },
 	{ "check-mailmap", cmd_check_mailmap, RUN_SETUP },
 	{ "check-ref-format", cmd_check_ref_format },
-	{ "checkout", cmd_checkout, RUN_SETUP | NEED_WORK_TREE },
+	{ "checkout", cmd_checkout, RUN_SETUP },
 	{ "checkout-index", cmd_checkout_index,
 		RUN_SETUP | NEED_WORK_TREE},
 	{ "cherry", cmd_cherry, RUN_SETUP },

t/t2025-checkout-to.sh

diff --git a/t/t2025-checkout-to.sh b/t/t2025-checkout-to.sh
index e2db078..4bd1df4 100755
--- a/t/t2025-checkout-to.sh
+++ b/t/t2025-checkout-to.sh
@@ -81,4 +81,19 @@
 	)
 '
 
+test_expect_success 'checkout --to from a bare repo' '
+	(
+		git clone --bare . bare &&
+		cd bare &&
+		git checkout --to ../there2 -b bare-master master
+	)
+'
+
+test_expect_success 'checkout from a bare repo without --to' '
+	(
+		cd bare &&
+		test_must_fail git checkout master
+	)
+'
+
 test_done