reflog existence & reftable

Hi there,

(splitting off from a code review of my test cleanups.)

Currently, reflogs are stored in .git/log/*. Git adds entries to the reflog only if the reflog already exists (See the log_ref_setup() function).

The current iteration of the reftable design has a unified key space of {refname,index-number} for reflog entries. This causes there to be no distinction between

  1) reflog is empty (.git/logs/blah is a 0-byte file)
  2) reflog does not exist (.git/logs/blah does not exist)

This trips up some current tests that make assumptions on reflog existence.

I don't know why one can tweak reflog to be written or not, but the current functionality will cause a change in operation with reftable. I see two ways forward:

1) Have different functionality in case of reftable: you cannot query
for the existence of reflogs, and writing reflogs doesn't depend on
the existence of a reflog.

2) Add a reflog existence feature to reftable. We could introduce a
magical reflog entry, which indicates that the reflog exists (but
might be empty). This adds some complexity to the C code, but lets us
maintain backward compatibility.

What do you think?