get_worktrees() must return main worktree as first item even on error

This is required by git-worktree.txt, stating that the main worktree is
the first line (especially in --porcelain mode when we can't just change
behavior at will).

There's only one case when get_worktrees() may skip main worktree, when
parse_ref() fails. Update the code so that we keep first item as main
worktree and return something sensible in this case:

 - In user-friendly mode, since we're not constraint by anything,
   returning "(error)" should do the job (we already show "(detached
   HEAD)" which is not machine-friendly). Actually errors should be
   printed on stderr by parse_ref() (*)

 - In plumbing mode, we do not show neither 'bare', 'detached' or
   'branch ...', which is possible by the format description if I read
   it right.

Careful readers may realize that when the local variable "head_ref" in
get_main_worktree() is emptied, add_head_info() will do nothing to
wt->head_sha1. But that's ok because head_sha1 is zero-ized in the
previous patch.

(*) Well, it does not. But it's supposed to be a stop gap implementation
    until we can reuse refs code to parse "ref: " stuff in HEAD, from
    resolve_refs_unsafe(). Now may be the time since refs refactoring is
    mostly done.

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

1 files changed, 4 insertions, 2 deletions

diff --git a/builtin/worktree.c b/builtin/worktree.c
index 8a654e4..b835b91 100644
--- a/builtin/worktree.c
+++ b/builtin/worktree.c
@@ -388,7 +388,7 @@ static void show_worktree_porcelain(struct worktree *wt)
 		printf("HEAD %s\n", sha1_to_hex(wt->head_sha1));
 		if (wt->is_detached)
 			printf("detached\n");
-		else
+		else if (wt->head_ref)
 			printf("branch %s\n", wt->head_ref);
 	}
 	printf("\n");
@@ -408,8 +408,10 @@ static void show_worktree(struct worktree *wt, int path_maxlen, int abbrev_len)
 				find_unique_abbrev(wt->head_sha1, DEFAULT_ABBREV));
 		if (wt->is_detached)
 			strbuf_addstr(&sb, "(detached HEAD)");
-		else
+		else if (wt->head_ref)
 			strbuf_addf(&sb, "[%s]", shorten_unambiguous_ref(wt->head_ref, 0));
+		else
+			strbuf_addstr(&sb, "(error)");
 	}
 	printf("%s\n", sb.buf);