Re: [PATCH v6 2/3] branch: report invalid tracking branch as broken

Jiang Xin [off-list ref] writes:

If a branch has been set to track a upstream, but the upstream branch
is missing or invalid, the tracking info is silently ignored in the
output of some commands such as "git branch -vv" and "git status",
as if there were no such tracking settings.

Junio suggested broken upstream should be reported [1]. E.g.

    $ git branch -v -v
      master    e67ac84 initial
    * topic     3fc0f2a [topicbase: broken] topic

I'd assume this is s/broken/gone/ to match what the rest of the log
message says?

    $ git status
    # On branch topic
    # Your branch is based on a broken ref 'topicbase'.
    #   (use "git branch --unset-upstream" to fixup)
    ...

    $ git status -b -s
    ## topic...topicbase [broken]
    ...

In order to do like that, we need to distinguish these three cases
(i.e. no tracking, with configured but no longer valid tracking, and
with tracking) in function stat_tracking_info(). So the refactored
function stat_tracking_info() has three return values: -1 (with "gone"
base), 0 (no base), and 1 (with base).