test-ref-store: tweaks to for-each-reflog-ent format

test-ref-store: tweaks to for-each-reflog-ent format

We have some tests that read from files in .git/logs/ hierarchy
when checking if correct reflog entries are created, but that is
too specific to the files backend.  Other backends like reftable
may not store its reflog entries in such a "one line per entry"
format.

Update for-each-reflog-ent test helper to produce output that
is identical to lines in a reflog file files backend uses.
That way, (1) the current tests can be updated to use the test
helper to read the reflog entries instead of (parts of) reflog
files, and perform the same inspection for correctness, and (2)
when the ref backend is swapped to another backend, the updated
test can be used as-is to check the correctness.

Adapt t1400 to use the for-each-reflog-ent test helper.

Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>