status: fix branch shown when not only bisecting

status: fix branch shown when not only bisecting

In 83c750acde (wt-status.*: better advice for git status added,
2012-06-05), git-status received new informative messages to describe
the ongoing work in a worktree.

These messages were enhanced in 0722c805d6 (status: show the branch name
if possible in in-progress info, 2013-02-03), to show, if possible, the
branch where the operation was initiated.

Since then, we show incorrect information when several operations are in
progress and one of them is bisect:

   $ git checkout -b foo
   $ GIT_SEQUENCE_EDITOR='echo break >' git rebase -i HEAD~
   $ git checkout -b bar
   $ git bisect start
   $ git status
   ...

   You are currently editing a commit while rebasing branch 'bar' on '...'.

   You are currently bisecting, started from branch 'bar'.

   ...

Note that we erroneously say "while rebasing branch 'bar'" when we
should be referring to "foo".

This must have gone unnoticed for so long because it must be unusual to
start a bisection while another operation is in progress.  And even less
usual to involve different branches.

It caught my attention reviewing a leak introduced in 8b87cfd000
(wt-status: move strbuf into read_and_strip_branch(), 2013-03-16).

A simple change to deal with this situation can be to record in struct
wt_status_state, the branch where the bisect starts separately from the
branch related to other operations.

Let's do it and so we'll be able to display correct information and
we'll avoid the leak as well.

Signed-off-by: Rubén Justo <rjusto@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>