reftable/stack: gracefully handle failed auto-compaction due to locks

reftable/stack: gracefully handle failed auto-compaction due to locks

Whenever we commit a new table to the reftable stack we will end up
invoking auto-compaction of the stack to keep the total number of tables
at bay. This auto-compaction may fail though in case at least one of the
tables which we are about to compact is locked. This is indicated by the
compaction function returning `REFTABLE_LOCK_ERROR`. We do not handle
this case though, and thus bubble that return value up the calling
chain, which will ultimately cause a failure.

Fix this bug by ignoring `REFTABLE_LOCK_ERROR`.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>