builtin/refs: get worktrees without reading head information

In "packed-backend.c", there are some functions such as "create_snapshot"
and "next_record" which would check the correctness of the content of
the "packed-ref" file. When anything is bad, the program will die.

It may seem that we have nothing relevant to above feature, because we
are going to read and parse the raw "packed-ref" file without creating
the snapshot and using the ref iterator to check the consistency.

However, when using "get_worktrees" in "builtin/refs", we would parse
the "HEAD" information. If the referent of the "HEAD" is inside the
"packed-ref", we will call "create_snapshot" function to parse the
"packed-ref" to get the information. No matter whether the entry of
"HEAD" in "packed-ref" is correct, "create_snapshot" would call
"verify_buffer_safe" to check whether there is a newline in the last
line of the file. If not, the program will die.

Although this behavior has no harm for the program, it will
short-circuit the program. When the users execute "git refs verify" or
"git fsck", we should avoid reading the head information, which may
execute the read operation in packed backend with stricter checks to die
the program. Instead, we should continue to check other parts of the
"packed-refs" file completely.

Fortunately, in 465a22b338 (worktree: skip reading HEAD when repairing
worktrees, 2023-12-29), we have introduced a function
"get_worktrees_internal" which allows us to get worktrees without
reading head information.

Create a new exposed function "get_worktrees_without_reading_head", then
replace the "get_worktrees" in "builtin/refs" with the new created
function.

Mentored-by: Patrick Steinhardt <ps@pks.im>
Mentored-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: shejialuo <shejialuo@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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