worktree: add --detach option

One of git-worktree's roles is to populate the new worktree, much like
git-checkout, and thus, for convenience, ought to support several of the
same shortcuts. Toward this goal, add a --detach option to detach HEAD
in the new worktree.

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

Documentation/git-worktree.txt

@@ -9,7 +9,7 @@ git-worktree - Manage multiple worktrees
 SYNOPSIS
 --------
 [verse]
-'git worktree add' [-f] <path> <branch>
+'git worktree add' [-f] [--detach] <path> <branch>
 'git worktree prune' [-n] [-v] [--expire <expire>]
 
 DESCRIPTION
@@ -64,6 +64,10 @@ OPTIONS
 	is already checked out by another worktree. This option overrides
 	that safeguard.
 
+--detach::
+	With `add`, detach HEAD in the new worktree. See "DETACHED HEAD" in
+	linkgit:git-checkout[1].
+
 -n::
 --dry-run::
 	With `prune`, do not remove anything; just report what it would

builtin/worktree.c

@@ -125,11 +125,12 @@ static int prune(int ac, const char **av, const char *prefix)
 static int add(int ac, const char **av, const char *prefix)
 {
 	struct child_process c;
-	int force = 0;
+	int force = 0, detach = 0;
 	const char *path, *branch;
 	struct argv_array cmd = ARGV_ARRAY_INIT;
 	struct option options[] = {
 		OPT__FORCE(&force, N_("checkout <branch> even if already checked out in other worktree")),
+		OPT_BOOL(0, "detach", &detach, N_("detach HEAD at named commit")),
 		OPT_END()
 	};
 
@@ -144,6 +145,8 @@ static int add(int ac, const char **av, const char *prefix)
 	argv_array_pushl(&cmd, "--to", path, NULL);
 	if (force)
 		argv_array_push(&cmd, "--ignore-other-worktrees");
+	if (detach)
+		argv_array_push(&cmd, "--detach");
 	argv_array_push(&cmd, branch);
 
 	memset(&c, 0, sizeof(c));