bisect: remove Cogito-related code

Once upon a time, there was this idea that Git would not actually be a
single coherent program, but rather a set of low-level programs that
users cobble together via shell scripts, or develop high-level user
interfaces for Git, or both.

Cogito was such a high-level user interface, incidentally implemented
via shell scripts that cobble together Git calls.

It did turn out relatively quickly that Git would much rather provide a
useful high-level user interface itself.

As of April 19th, 2007, Cogito was therefore discontinued (see
https://lore.kernel.org/git/[email protected]/).

Nevertheless, for almost 15 years after that announcement, Git carried
special code in `git bisect` to accommodate Cogito.

Since it is beyond doubt that there are no more Cogito users, let's
remove the last remnant of Cogito-accommodating code.

Signed-off-by: Johannes Schindelin <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: dscho

builtin/bisect.c

@@ -14,7 +14,6 @@ static GIT_PATH_FUNC(git_path_bisect_expected_rev, "BISECT_EXPECTED_REV")
 static GIT_PATH_FUNC(git_path_bisect_ancestors_ok, "BISECT_ANCESTORS_OK")
 static GIT_PATH_FUNC(git_path_bisect_start, "BISECT_START")
 static GIT_PATH_FUNC(git_path_bisect_log, "BISECT_LOG")
-static GIT_PATH_FUNC(git_path_head_name, "head-name")
 static GIT_PATH_FUNC(git_path_bisect_names, "BISECT_NAMES")
 static GIT_PATH_FUNC(git_path_bisect_first_parent, "BISECT_FIRST_PARENT")
 static GIT_PATH_FUNC(git_path_bisect_run, "BISECT_RUN")
@@ -808,13 +807,6 @@ static enum bisect_error bisect_start(struct bisect_terms *terms, const char **a
 			strbuf_addstr(&start_head, oid_to_hex(&head_oid));
 		} else if (!get_oid(head, &head_oid) &&
 			   skip_prefix(head, "refs/heads/", &head)) {
-			/*
-			 * This error message should only be triggered by
-			 * cogito usage, and cogito users should understand
-			 * it relates to cg-seek.
-			 */
-			if (!is_empty_or_missing_file(git_path_head_name()))
-				return error(_("won't bisect on cg-seek'ed tree"));
 			strbuf_addstr(&start_head, head);
 		} else {
 			return error(_("bad HEAD - strange symbolic ref"));