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>

1 files changed, 16 insertions, 0 deletions

diff --git a/builtin/worktree.c b/builtin/worktree.c
index ea9678cac8..7021d02585 100644
--- a/builtin/worktree.c
+++ b/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;