worktree: make setup of new HEAD distinct from worktree population

git-worktree currently conflates setting of HEAD in the new worktree and
initial worktree population into a single git-checkout invocation which
requires git-checkout to have special knowledge that it is operating on
a newly created worktree. The eventual goal is to rid git-checkout of
that overly-intimate knowledge.

Once these operations are separate, git-worktree will no longer be able
to delegate to git-branch the setting of the new worktree's HEAD to the
desired branch (or commit, if detached). Therefore, make git-worktree
itself responsible for setting up HEAD as either a symbolic reference,
if associated with a branch, or detached, if not.

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

Author: sunshineco

builtin/worktree.c

@@ -278,12 +278,21 @@ static int add_worktree(const char *path, const char *refname,
 	argv_array_pushf(&child_env, "%s=%s", GIT_WORK_TREE_ENVIRONMENT, path);
 	memset(&cp, 0, sizeof(cp));
 	cp.git_cmd = 1;
+
+	if (commit)
+		argv_array_pushl(&cp.args, "update-ref", "HEAD",
+				 sha1_to_hex(commit->object.sha1), NULL);
+	else
+		argv_array_pushl(&cp.args, "symbolic-ref", "HEAD",
+				 symref.buf, NULL);
+	cp.env = child_env.argv;
+	ret = run_command(&cp);
+	if (ret)
+		goto done;
+
+	cp.argv = NULL;
+	argv_array_clear(&cp.args);
 	argv_array_push(&cp.args, "checkout");
-	if (opts->force)
-		argv_array_push(&cp.args, "--ignore-other-worktrees");
-	if (opts->detach)
-		argv_array_push(&cp.args, "--detach");
-	argv_array_push(&cp.args, refname);
 	cp.env = child_env.argv;
 	ret = run_command(&cp);
 	if (!ret) {
@@ -293,6 +302,7 @@ static int add_worktree(const char *path, const char *refname,
 		junk_work_tree = NULL;
 		junk_git_dir = NULL;
 	}
+done:
 	strbuf_reset(&sb);
 	strbuf_addf(&sb, "%s/locked", sb_repo.buf);
 	unlink_or_warn(sb.buf);