packed-backend: check if header starts with "# pack-refs with: "

packed-backend: check if header starts with "# pack-refs with: "

We always write a space after "# pack-refs with:" but we don't align
with this rule in the "create_snapshot" method where we would check
whether header starts with "# pack-refs with:". It might seem that we
should undoubtedly tighten this rule, however, we don't have any
technical documentation about this and there is a possibility that we
would break the compatibility for other third-party libraries.

By investigating influential third-party libraries, we could conclude
how these libraries handle the header of "packed-refs" file:

1. libgit2 is fine and always writes the space. It also expects the
   whitespace to exist.
2. JGit does not expect th header to have a trailing space, but expects
   the "peeled" capability to have a leading space, which is mostly
   equivalent because that capability is typically the first one we
   write. It always writes the space.
3. gitoxide expects the space t exist and writes it.
4. go-git doesn't create the header by default.

As many third-party libraries expect a single space after "# pack-refs
with:", if we forget to write the space after the colon,
"create_snapshot" won't catch this. And we would break other
re-implementations. So, we'd better tighten the rule by checking whether
the header starts with "# pack-refs with: ".

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>