checkout: fix bug with --to and relative HEAD

Given "git checkout --to <path> HEAD~1", the new worktree's HEAD should
begin life at the current branch's HEAD~1, however, it actually ends up
at HEAD~2. This happens because:

    1. git-checkout resolves HEAD~1

    2. to satisfy is_git_directory(), prepare_linked_worktree() creates
       a HEAD for the new worktree with the value of the resolved HEAD~1

    3. git-checkout re-invokes itself with the same arguments within the
       new worktree to populate the worktree

    4. the sub git-checkout resolves HEAD~1 relative to its own HEAD,
       which is the resolved HEAD~1 from the original invocation,
       resulting unexpectedly and incorrectly in HEAD~2 (relative to the
       original)

Fix this by unconditionally assigning the current worktree's HEAD as the
value of the new worktree's HEAD.

As a side-effect, this change also eliminates a dependence within
prepare_linked_checkout() upon 'struct branch_info'. The plan is to
eventually relocate "git checkout --to" functionality to "git worktree
add", and worktree.c won't have knowledge of 'struct branch_info', so
removal of this dependency is a step toward that goal.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/checkout.c

@@ -863,6 +863,7 @@ static int prepare_linked_checkout(const struct checkout_opts *opts,
 	struct stat st;
 	struct child_process cp;
 	int counter = 0, len, ret;
+	unsigned char rev[20];
 
 	if (!new->commit)
 		die(_("no branch specified"));
@@ -920,13 +921,20 @@ static int prepare_linked_checkout(const struct checkout_opts *opts,
 		   real_path(get_git_common_dir()), name);
 	/*
 	 * This is to keep resolve_ref() happy. We need a valid HEAD
-	 * or is_git_directory() will reject the directory. Any valid
+	 * or is_git_directory() will reject the directory. Moreover, HEAD
-	 * value would do because this value will be ignored and
+	 * in the new worktree must resolve to the same value as HEAD in
-	 * replaced at the next (real) checkout.
+	 * the current tree since the command invoked to populate the new
+	 * worktree will be handed the branch/ref specified by the user.
+	 * For instance, if the user asks for the new worktree to be based
+	 * at HEAD~5, then the resolved HEAD~5 in the new worktree must
+	 * match the resolved HEAD~5 in the current tree in order to match
+	 * the user's expectation.
 	 */
+	if (!resolve_ref_unsafe("HEAD", 0, rev, NULL))
+		die(_("unable to resolve HEAD"));
 	strbuf_reset(&sb);
 	strbuf_addf(&sb, "%s/HEAD", sb_repo.buf);
-	write_file(sb.buf, 1, "%s\n", sha1_to_hex(new->commit->object.sha1));
+	write_file(sb.buf, 1, "%s\n", sha1_to_hex(rev));
 	strbuf_reset(&sb);
 	strbuf_addf(&sb, "%s/commondir", sb_repo.buf);
 	write_file(sb.buf, 1, "../..\n");

t/t2025-checkout-to.sh

@@ -134,4 +134,14 @@ test_expect_success 'checkout with grafts' '
 	test_cmp expected actual
 '
 
+test_expect_success 'checkout --to from relative HEAD' '
+	test_commit a &&
+	test_commit b &&
+	test_commit c &&
+	git rev-parse HEAD~1 >expected &&
+	git checkout --to relhead HEAD~1 &&
+	git -C relhead rev-parse HEAD >actual &&
+	test_cmp expected actual
+'
+
 test_done