packed-backend: add "packed-refs" header consistency check

In "packed-backend.c::create_snapshot", if there is a header (the line
which starts with '#'), we will check whether the line starts with "#
pack-refs with: ". However, we need to consider other situations and
discuss whether we need to add checks.

1. If the header does not exist, we should not report an error to the
   user. This is because in older Git version, we never write header in
   the "packed-refs" file. Also, we do allow no header in "packed-refs"
   in runtime.
2. If the header content does not start with "# packed-ref with: ", we
   should report an error just like what "create_snapshot" does. So,
   create a new fsck message "badPackedRefHeader(ERROR)" for this.
3. If the header content is not the same as the constant string
   "PACKED_REFS_HEADER". This is expected because we make it extensible
   intentionally and runtime "create_snapshot" won't complain about
   unknown traits. In order to align with the runtime behavior. There is
   no need to report.

As we have analyzed, we only need to check the case 2 in the above. In
order to do this, use "open_nofollow" function to get the file
descriptor and then read the "packed-refs" file via "strbuf_read". Like
what "create_snapshot" and other functions do, we could split the line
by finding the next newline in the buffer. When we cannot find a
newline, we could report an error.

So, create a function "packed_fsck_ref_next_line" to find the next
newline and if there is no such newline, use
"packedRefEntryNotTerminated(ERROR)" to report an error to the user.

Then, parse the first line to apply the checks. Update the test to
exercise the code.

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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