Re: [PATCH v4] for-each-ref: Always check stat_tracking_info()'s return value.

Raphael Kubo da Costa [off-list ref] writes:

The code handling %(upstream:track) and %(upstream:trackshort) assumed
it always had a valid branch that had been sanitized earlier in
populate_value(), and thus did not check the return value of the call to
stat_tracking_info().

While there is indeed some sanitization code that basically corresponds
to stat_tracking_info() returning 0 (no base branch set), the function
can also return -1 when the base branch did exist but has since then
been deleted.

In this case, num_ours and num_theirs had undefined values and a call to
`git for-each-ref --format="%(upstream:track)"` could print spurious
values such as

  [behind -111794512]
  [ahead 38881640, behind 5103867]

even for repositories with one single commit.

We now properly verify stat_tracking_info()'s return value and do not
print anything if it returns -1. This behavior also matches the
documentation ("has no effect if the ref does not have tracking
information associated with it").

Helped-by: Eric Sunshine [off-list ref]
Helped-by: Jeff King [off-list ref]
Signed-off-by: Raphael Kubo da Costa <redacted>
---
v4: Use Jeff's suggestion and simplify the test case and making it
    easier to understand what is actually being tested.

Nicely done, indeed.  Thanks all.  Will queue with some cosmetic
tweaks.