projects: set receive.denyDeleteCurrent to warn

projects: set receive.denyDeleteCurrent to warn

Originally Girocco allowed deletion of the branch pointed to by the
symbolic ref HEAD on a push.  However, it was eventually overcome by Git
changes and Git version updates so that it now emits a very long and
ugly message instead and denies the action.

Since non fast-forward updates have always (and continue to be) allowed,
there's no reason to deny deleting the current branch since a non
fast-forward update to an empty tree can be pushed instead to accomplish
essentially the same thing.

Set receive.denyDeleteCurrent to warn to avoid the long ugly message but
continue to notify the client that further action might be needed (such
as changing the default branch).

The other possible values are false or ignore which allow the deletion
without any message at all and true or refuse which denys the deletion
with a brief message.  The value warn seems like the best choice since
it allows the action while notifying the client.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>