setup: use the default algorithm to initialize repo format

When we define a new repository format with REPOSITORY_FORMAT_INIT, we
always use GIT_HASH_SHA1, and this value ends up getting used as the
default value to initialize a repository if none of the command line,
environment, or config tell us to do otherwise.

Because we might not always want to use SHA-1 as the default, let's
instead specify the default hash algorithm constant so that we will use
whatever the specified default is.

However, we also need to continue to read older repositories.  If we're
in a v0 repository or extensions.objectformat is not set, then we must
continue to default to the original hash algorithm: SHA-1.  If an
algorithm is set explicitly, however, it will override the hash_algo
member of the repository_format struct and we'll get the right value.

Similarly, if the repository was initialized before Git 0.99.3, then it
may lack a core.repositoryformatversion key, and some repositories lack
a config file altogether.  In both cases, format->version is -1 and we
need to assume that SHA-1 is in use.

Because clear_repository_format reinitializes the struct
repository_format and therefore sets the hash_algo member to the default
(which could in the future not be SHA-1), we need to reset this member
explicitly.  We know, however, that at the point we call
read_repository_format, we are actually reading an existing repository
and not initializing a new one or operating outside of a repository, so
we are not changing the default behavior back to SHA-1 if the default
algorithm is different.

It is potentially questionable that we ignore all repository
configuration if there is a config file but it doesn't have
core.repositoryformatversion set, in which case we reset all of the
configuration to the default.  However, it is unclear what the right
thing to do instead with such an old repository is and a simple git init
will add the missing entry, so for now, we simply honor what the
existing code does and reset the value to the default, simply adding our
initialization to SHA-1.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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