builtin/init-db: allow specifying hash algorithm on command line
Allow the user to specify the hash algorithm on the command line by
using the --object-format option to git init. Validate that the user is
not attempting to reinitialize a repository with a different hash
algorithm. Ensure that if we are writing a non-SHA-1 repository that we
set the repository version to 1 and write the objectFormat extension.
Restrict this option to work only when ENABLE_SHA256 is set until the
codebase is in a situation to fully support this.
Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>