Jeff King [off-list ref] writes:

with an empty reflog file (added by that same commit). The code in
get_oid_basic() to parse reflogs doesn't suffer from the same bugs: it
checks up front that the reflog file exists, it preloads the output oid
with the current ref value, and it doesn't look at other fields (like
the reflog message).

It is a usability hack to allow foo@{0} to resolve successfully,
instead of causing a failure, when there is no reflog entries for
foo, I would think.  As to the "show-branch -g", the intent is to
see the recent evolution of the branch, so I am fine if we do not
give any output when no reflog entries exist (i.e. "no evolution
behind the current state---it just is"), or just one entry output
for "foo@{0}" to say "there is only one recent state".

Even though it may feel wrong to successfully resolve foo@{0} when
reflog for foo does not exist at the mechanical level (read: the
implementors of reflog mechanism may find the usability hack a bad
idea), I suspect at the end-user level it may be closer to what
people expect out of foo@{0} (i.e. "give me the latest").

Thanks.