cmd/go: add support for git sha256 hashes

Git's supported SHA 256 object hashes since 2.29[1] in 2021, and Gitlab
now has experimental support for sha256 repos.

Take rsc@'s suggestion of checking the of the length of the hashes from
git ls-remote to determine whether a git repo is using sha256 hashes and
decide whether to pass --object-format=sha256 to git init.

Unfortunately, just passing --object-format=sha256 wasn't quite enough,
though. We also need to decide whether the hash-length is 64 hex bytes
or 40 hex bytes when resolving refs to decide whether we've been passed
a full commit-hash. To that end, we use
git config extensions.objectformat to decide whether the (now guaranteed
local) repo is using sha256 hashes and hence 64-hex-byte strings.

[1]: lost experimental status in 2.42 from Aug 2023
(https://lore.kernel.org/git/xmqqr0nwp8mv.fsf@gitster.g/)

For: #68359
Change-Id: I47f480ab8334128c5d17570fe76722367d0d8ed8
Reviewed-on: https://go-review.googlesource.com/c/go/+/636475
Auto-Submit: Sam Thanawalla <samthanawalla@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Sam Thanawalla <samthanawalla@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Michael Matloob <matloob@google.com>
Reviewed-by: David Finkel <david.finkel@gmail.com>

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