worktree: introduce "add" command

The plan is to relocate "git checkout --to" functionality to "git
worktree add". As a first step, introduce a bare-bones git-worktree
"add" command along with documentation. At this stage, "git worktree
add" merely invokes "git checkout --to" behind the scenes, but an
upcoming patch will move the actual functionality
(checkout.c:prepare_linked_checkout() and its helpers) to worktree.c.

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

Documentation/git-worktree.txt

@@ -9,13 +9,13 @@ git-worktree - Manage multiple worktrees
 SYNOPSIS
 --------
 [verse]
+'git worktree add' <path> <branch>
 'git worktree prune' [-n] [-v] [--expire <expire>]
 
 DESCRIPTION
 -----------
 
-Manage multiple worktrees attached to the same repository. These are
+Manage multiple worktrees attached to the same repository.
-created by the command `git checkout --to`.
 
 A git repository can support multiple working trees, allowing you to check
 out more than one branch at a time.  With `git checkout --to` a new working
@@ -45,6 +45,12 @@ pruning should be suppressed. See section "DETAILS" for more information.
 
 COMMANDS
 --------
+add <path> <branch>::
+
+Create `<path>` and checkout `<branch>` into it. The new working directory
+is linked to the current repository, sharing everything except working
+directory specific files such as HEAD, index, etc.
+
 prune::
 
 Prune working tree information in $GIT_DIR/worktrees.
@@ -118,7 +124,7 @@ refactoring session.
 
 ------------
 $ git branch emergency-fix master
-$ git checkout --to ../temp emergency-fix
+$ git worktree add ../temp emergency-fix
 $ pushd ../temp
 # ... hack hack hack ...
 $ git commit -a -m 'emergency fix for boss'
@@ -133,9 +139,8 @@ Multiple checkout support for submodules is incomplete. It is NOT
 recommended to make multiple checkouts of a superproject.
 
 git-worktree could provide more automation for tasks currently
-performed manually or via other commands, such as:
+performed manually, such as:
 
-- `add` to create a new linked worktree
 - `remove` to remove a linked worktree and its administrative files (and
   warn if the worktree is dirty)
 - `mv` to move or rename a worktree and update its administrative files
@@ -143,11 +148,6 @@ performed manually or via other commands, such as:
 - `lock` to prevent automatic pruning of administrative files (for instance,
   for a worktree on a portable device)
 
-SEE ALSO
---------
-
-linkgit:git-checkout[1]
-
 GIT
 ---
 Part of the linkgit:git[1] suite

builtin/worktree.c

@@ -2,8 +2,11 @@
 #include "builtin.h"
 #include "dir.h"
 #include "parse-options.h"
+#include "argv-array.h"
+#include "run-command.h"
 
 static const char * const worktree_usage[] = {
+	N_("git worktree add <path> <branch>"),
 	N_("git worktree prune [<options>]"),
 	NULL
 };
@@ -119,6 +122,32 @@ static int prune(int ac, const char **av, const char *prefix)
 	return 0;
 }
 
+static int add(int ac, const char **av, const char *prefix)
+{
+	struct child_process c;
+	const char *path, *branch;
+	struct argv_array cmd = ARGV_ARRAY_INIT;
+	struct option options[] = {
+		OPT_END()
+	};
+
+	ac = parse_options(ac, av, prefix, options, worktree_usage, 0);
+	if (ac != 2)
+		usage_with_options(worktree_usage, options);
+
+	path = prefix ? prefix_filename(prefix, strlen(prefix), av[0]) : av[0];
+	branch = av[1];
+
+	argv_array_push(&cmd, "checkout");
+	argv_array_pushl(&cmd, "--to", path, NULL);
+	argv_array_push(&cmd, branch);
+
+	memset(&c, 0, sizeof(c));
+	c.git_cmd = 1;
+	c.argv = cmd.argv;
+	return run_command(&c);
+}
+
 int cmd_worktree(int ac, const char **av, const char *prefix)
 {
 	struct option options[] = {
@@ -127,6 +156,8 @@ int cmd_worktree(int ac, const char **av, const char *prefix)
 
 	if (ac < 2)
 		usage_with_options(worktree_usage, options);
+	if (!strcmp(av[1], "add"))
+		return add(ac - 1, av + 1, prefix);
 	if (!strcmp(av[1], "prune"))
 		return prune(ac - 1, av + 1, prefix);
 	usage_with_options(worktree_usage, options);