setup: make object format configurable via config

The object format for repositories can either be configured explicitly
by passing the `--object-format=` option to git-init(1) or git-clone(1),
or globally by setting the `GIT_DEFAULT_HASH` environment variable.
While the former makes sense, setting random environment variables is
not really a good user experience in case someone decides to only use
SHA256 repositories.

It is only natural to expect for a user that things like this can also
be configured via their config. As such, introduce a new config
"init.defaultObjectFormat", similar to "init.defaultBranch", that allows
the user to configure the default object format when creating new repos.

The precedence order now is the following, where the first one wins:

  1. The `--object-format=` switch.

  2. The `GIT_DEFAULT_HASH` environment variable.

  3. The `init.defaultObjectFormat` config variable.

This matches the typical precedence order we use in Git. We typically
let the environment override the config such that the latter can easily
be overridden on an ephemeral basis, for example by scripts.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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