init-db: check template and repository format.

init-db: check template and repository format.

This makes init-db repository version aware.

It checks if an existing config file says the repository being
reinitialized is of a wrong version and aborts before doing
further harm.

When copying the templates, it makes sure the they are of the
right repository format version.  Otherwise the templates are
ignored with an warning message.

It copies the templates before creating the HEAD, and if the
config file is copied from the template directory, reads it,
primarily to pick up the value of core.symrefsonly.

It changes the way the result of the filemode reliability test
is written to the configuration file using git_config_set().
The test is done even if the config file was copied from the
templates.

And finally, our own repository format version is written to the
config file.

Signed-off-by: Junio C Hamano <junkio@cox.net>