worktree: make add <path> <branch> dwim

Currently 'git worktree add <path> <branch>', errors out when 'branch'
is not a local branch.  It has no additional dwim'ing features that one
might expect.

Make it behave more like 'git checkout <branch>' when the branch doesn't
exist locally, but a remote tracking branch uniquely matches the desired
branch name, i.e. create a new branch from the remote tracking branch
and set the upstream to the remote tracking branch.

As 'git worktree add' currently just dies in this situation, there are
no backwards compatibility worries when introducing this feature.

Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-worktree.txt

@@ -52,6 +52,14 @@ is linked to the current repository, sharing everything except working
 directory specific files such as HEAD, index, etc. `-` may also be
 specified as `<commit-ish>`; it is synonymous with `@{-1}`.
 +
+If <commit-ish> is a branch name (call it `<branch>` and is not found,
+and neither `-b` nor `-B` nor `--detach` are used, but there does
+exist a tracking branch in exactly one remote (call it `<remote>`)
+with a matching name, treat as equivalent to
+------------
+$ git worktree add --track -b <branch> <path> <remote>/<branch>
+------------
++
 If `<commit-ish>` is omitted and neither `-b` nor `-B` nor `--detach` used,
 then, as a convenience, a new branch based at HEAD is created automatically,
 as if `-b $(basename <path>)` was specified.

builtin/worktree.c

@@ -1,4 +1,5 @@
 #include "cache.h"
+#include "checkout.h"
 #include "config.h"
 #include "builtin.h"
 #include "dir.h"
@@ -390,6 +391,21 @@ static int add(int ac, const char **av, const char *prefix)
 		opts.new_branch = xstrndup(s, n);
 	}
 
+	if (ac == 2 && !opts.new_branch && !opts.detach) {
+		struct object_id oid;
+		struct commit *commit;
+		const char *remote;
+
+		commit = lookup_commit_reference_by_name(branch);
+		if (!commit) {
+			remote = unique_tracking_name(branch, &oid);
+			if (remote) {
+				opts.new_branch = branch;
+				branch = remote;
+			}
+		}
+	}
+
 	if (opts.new_branch) {
 		struct child_process cp = CHILD_PROCESS_INIT;
 		cp.git_cmd = 1;

t/t2025-worktree-add.sh

@@ -365,4 +365,23 @@ test_expect_success '--no-track avoids setting up tracking' '
 	)
 '
 
+test_expect_success '"add" <path> <non-existent-branch> fails' '
+	test_must_fail git worktree add foo non-existent
+'
+
+test_expect_success '"add" <path> <branch> dwims' '
+	test_when_finished rm -rf repo_upstream repo_dwim foo &&
+	setup_remote_repo repo_upstream repo_dwim &&
+	git init repo_dwim &&
+	(
+		cd repo_dwim &&
+		git worktree add ../foo foo
+	) &&
+	(
+		cd foo &&
+		test_branch_upstream foo repo_upstream foo &&
+		test_cmp_rev refs/remotes/repo_upstream/foo refs/heads/foo
+	)
+'
+
 test_done