checkout: add --ignore-other-wortrees

Noticed-by: Mark Levedahl <mlevedahl@gmail.com>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-checkout.txt

@@ -232,6 +232,12 @@ section of linkgit:git-add[1] to learn how to operate the `--patch` mode.
 	specific files such as HEAD, index... See "MULTIPLE WORKING
 	TREES" section for more information.
 
+--ignore-other-worktrees::
+	`git checkout` refuses when the wanted ref is already checked
+	out by another worktree. This option makes it check the ref
+	out anyway. In other words, the ref can be held by more than one
+	worktree.
+
 <branch>::
 	Branch to checkout; if it refers to a branch (i.e., a name that,
 	when prepended with "refs/heads/", is a valid ref), then that

builtin/checkout.c

@@ -37,6 +37,7 @@ struct checkout_opts {
 	int writeout_stage;
 	int overwrite_ignore;
 	int ignore_skipworktree;
+	int ignore_other_worktrees;
 
 	const char *new_branch;
 	const char *new_branch_force;
@@ -1210,7 +1211,8 @@ static int parse_branchname_arg(int argc, const char **argv,
 		int flag;
 		char *head_ref = resolve_refdup("HEAD", 0, sha1, &flag);
 		if (head_ref &&
-		    (!(flag & REF_ISSYMREF) || strcmp(head_ref, new->path)))
+		    (!(flag & REF_ISSYMREF) || strcmp(head_ref, new->path)) &&
+		    !opts->ignore_other_worktrees)
 			check_linked_checkouts(new);
 		free(head_ref);
 	}
@@ -1341,6 +1343,8 @@ int cmd_checkout(int argc, const char **argv, const char *prefix)
 				N_("second guess 'git checkout no-such-branch'")),
 		OPT_FILENAME(0, "to", &opts.new_worktree,
 			   N_("check a branch out in a separate working directory")),
+		OPT_BOOL(0, "ignore-other-worktrees", &opts.ignore_other_worktrees,
+			 N_("do not check if another worktree is holding the given ref")),
 		OPT_END(),
 	};
 

t/t2025-checkout-to.sh

@@ -79,6 +79,13 @@ test_expect_success 'die the same branch is already checked out' '
 	)
 '
 
+test_expect_success 'not die the same branch is already checked out' '
+	(
+		cd here &&
+		git checkout --ignore-other-worktrees --to anothernewmaster newmaster
+	)
+'
+
 test_expect_success 'not die on re-checking out current branch' '
 	(
 		cd there &&