submodule status: correct path handling in recursive submodules

submodule status: correct path handling in recursive submodules

The new test which is a replica of the previous test except
that it executes from a sub directory. Prior to this patch
the test failed by having too many '../' prefixed:

  --- expect	2016-03-29 19:02:33.087336115 +0000
  +++ actual	2016-03-29 19:02:33.359343311 +0000
  @@ -1,7 +1,7 @@
    b23f134787d96fae589a6b76da41f4db112fc8db ../nested1 (heads/master)
  -+25d56d1ddfb35c3e91ff7d8f12331c2e53147dcc ../nested1/nested2 (file2)
  - 5ec83512b76a0b8170b899f8e643913c3e9b72d9 ../nested1/nested2/nested3 (heads/master)
  - 509f622a4f36a3e472affcf28fa959174f3dd5b5 ../nested1/nested2/nested3/submodule (heads/master)
  ++25d56d1ddfb35c3e91ff7d8f12331c2e53147dcc ../../nested1/nested2 (file2)
  + 5ec83512b76a0b8170b899f8e643913c3e9b72d9 ../../../nested1/nested2/nested3 (heads/master)
  + 509f622a4f36a3e472affcf28fa959174f3dd5b5 ../../../../nested1/nested2/nested3/submodule (heads/master)
    0c90624ab7f1aaa301d3bb79f60dcfed1ec4897f ../sub1 (0c90624)
    0c90624ab7f1aaa301d3bb79f60dcfed1ec4897f ../sub2 (0c90624)
    509f622a4f36a3e472affcf28fa959174f3dd5b5 ../sub3 (heads/master)

The path code in question:
  displaypath=$(relative_path "$prefix$sm_path")
  prefix=$displaypath
  if recursive:
    eval cmd_status

That way we change `prefix` each iteration to contain another
'../', because of the the relative_path computation is done
on an already computed relative path.

We must call relative_path exactly once with `wt_prefix` non empty.
Further calls in recursive instances to to calculate the displaypath
already incorporate the correct prefix from before. Fix the issue by
clearing `wt_prefix` in recursive calls.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>