refs/reftable: add fsck check for checking the table name

refs/reftable: add fsck check for checking the table name

Add glue code in 'refs/reftable-backend.c' which calls the reftable
library to perform the fsck checks. Here we also map the reftable errors
to Git' fsck errors.

Introduce a check to validate table names for a given reftable stack.
Also add 'badReftableTableName' as a corresponding error within Git. The
reftable specification mentions:

  It suggested to use
  ${min_update_index}-${max_update_index}-${random}.ref as a naming
  convention.

So treat non-conformant file names as warnings.

While adding the fsck header to 'refs/reftable-backend.c', modify the
list to maintain lexicographical ordering.

Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>