init-db: move writing repo version into a function

init-db: move writing repo version into a function

When we perform a clone, we won't know the remote side's hash algorithm
until we've read the heads.  Consequently, we'll need to rewrite the
repository format version and hash algorithm once we know what the
remote side has.  Move the code that does this into its own function so
that we can call it from clone in the future.

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