This fixes some memory leaks in remote.c. Not urgent, as they are quite
old, but they are newly triggered in the test suite by Harald's
hn/status-compare-with-push topic. So I think we'd want to build that
topic on top of these.

The first two are just preparatory cleanups. Patch 3 fixes the leak that
Harald's series triggers (and adds its own test, of course). Patch 4 is
a hypothetical leak that I don't think can be triggered in practice (so
it's more of a cleanup).

  [1/4]: remote: return non-const pointer from error_buf()
  [2/4]: remote: drop const return of tracking_for_push_dest()
  [3/4]: remote: fix leak in branch_get_push_1() with invalid "simple" config
  [4/4]: remote: always allocate branch.push_tracking_ref

 remote.c                | 24 ++++++++++++++----------
 remote.h                |  2 +-
 t/for-each-ref-tests.sh |  9 +++++++++
 3 files changed, 24 insertions(+), 11 deletions(-)

-Peff