reftable: add code to facilitate consistency checks

reftable: add code to facilitate consistency checks

The `git refs verify` command is used to run consistency checks on the
reference backends. This command is also invoked when users run 'git
fsck'. While the files-backend has some fsck checks added, the reftable
backend lacks such checks. Let's add the required infrastructure and a
check to test for the files present in the reftable directory.

Since the reftable library is treated as an independent library we
should ensure that the library code works independently without
knowledge about Git's internals. To do this, add both 'reftable/fsck.c'
and 'reftable/reftable-fsck.h'. Which provide an entry point
'reftable_fsck_check' for running fsck checks over a provided reftable
stack. The callee provides the function with callbacks to handle issue
and information reporting.

The added check, goes over all tables in the reftable stack validates
that they have a valid name. It not, it raises an error.

While here, move 'reftable/error.o' in the Makefile to retain
lexicographic ordering.

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