worktree: add --force option

By default, "git worktree add" refuses to create a new worktree when
the requested branch is already checked out elsewhere. Add a --force
option to override this safeguard.

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' <path> <branch>
+'git worktree add' [-f] <path> <branch>
 'git worktree prune' [-n] [-v] [--expire <expire>]
 
 DESCRIPTION
@@ -58,6 +58,12 @@ Prune working tree information in $GIT_DIR/worktrees.
 OPTIONS
 -------
 
+-f::
+--force::
+	By default, `add` refuses to create a new worktree when	`<branch>`
+	is already checked out by another worktree. This option overrides
+	that safeguard.
+
 -n::
 --dry-run::
 	With `prune`, do not remove anything; just report what it would

builtin/worktree.c

@@ -6,7 +6,7 @@
 #include "run-command.h"
 
 static const char * const worktree_usage[] = {
-	N_("git worktree add <path> <branch>"),
+	N_("git worktree add [<options>] <path> <branch>"),
 	N_("git worktree prune [<options>]"),
 	NULL
 };
@@ -125,9 +125,11 @@ 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;
 	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_END()
 	};
 
@@ -140,6 +142,8 @@ static int add(int ac, const char **av, const char *prefix)
 
 	argv_array_push(&cmd, "checkout");
 	argv_array_pushl(&cmd, "--to", path, NULL);
+	if (force)
+		argv_array_push(&cmd, "--ignore-other-worktrees");
 	argv_array_push(&cmd, branch);
 
 	memset(&c, 0, sizeof(c));