[PATCH v3] push: fix --force-if-includes when remote-tracking ref has no reflog

Since 99a1f9ae10 (push: add reflog check for "--force-if-includes", 2020-10-03), is_reachable_in_reflog() stops walking the reflog of the local branch at entries older than the newest reflog entry of the remote-tracking ref. That timestamp is read by a callback of refs_for_each_reflog_ent_reverse(), so when the remote-tracking ref has no reflog, the variable that holds the timestamp stays uninitialized.

With the files backend a remote-tracking ref created by "git clone" has no reflog and does not get one until it moves. On my machine the leftover value exceeds any real timestamp: the walk stops at the very first entry, never reaches the "Created from" entry that "checkout --track" wrote, and the push is rejected with "remote ref updated since checkout" although nothing on the remote has changed.

The cut-off is an optimization that rests on an assumption: an entry older than the moment the remote-tracking ref last moved is not expected to be the one being looked for. Without a reflog there is no such moment, hence no cut-off to apply. Initialize the timestamp to zero to say exactly that: timestamp_t is unsigned, so no entry compares older than zero and the comparison never fires. Using "now", or any fixed age, would instead cut the walk off at the first entry older than that bound, which is how the failure happens in the first place. The price is paid only when no matching entry is found: the walk then reaches the oldest entry and falls back to the merge-base check over what it collected, where the cut-off would have stopped it earlier.

Signed-off-by: Aleksei Sviridkin <f@lex.la>
---
Changes since v2:
  - reworded the first paragraph as you suggested
  - explain why zero is the fallback rather than "now" or a fixed age
  - dropped the Assisted-by trailer

 remote.c            |  2 +-
 t/t5533-push-cas.sh | 18 ++++++++++++++++++
 2 files changed, 19 insertions(+), 1 deletion(-)