t7060: add test for status --branch on a detached HEAD

t7060: add test for status --branch on a detached HEAD

This test fails when run under Valgrind because branch_get() gets passed
a bogus branch name pointer:

==62831== Invalid read of size 1
==62831==    at 0x4F76AE: branch_get (remote.c:1650)
==62831==    by 0x53499E: wt_shortstatus_print_tracking (wt-status.c:1654)
==62831==    by 0x53499E: wt_shortstatus_print (wt-status.c:1706)
==62831==    by 0x428D29: cmd_status (commit.c:1384)
==62831==    by 0x405D6D: run_builtin (git.c:350)
==62831==    by 0x405D6D: handle_builtin (git.c:536)
==62831==    by 0x404F10: run_argv (git.c:582)
==62831==    by 0x404F10: main (git.c:690)
==62831==  Address 0x5e89b0b is 6 bytes after a block of size 5 alloc'd
==62831==    at 0x4C28C4F: malloc (vg_replace_malloc.c:299)
==62831==    by 0x59579E9: strdup (strdup.c:42)
==62831==    by 0x52E108: xstrdup (wrapper.c:43)
==62831==    by 0x5322A6: wt_status_prepare (wt-status.c:130)
==62831==    by 0x4276E0: status_init_config (commit.c:184)
==62831==    by 0x428BB8: cmd_status (commit.c:1350)
==62831==    by 0x405D6D: run_builtin (git.c:350)
==62831==    by 0x405D6D: handle_builtin (git.c:536)
==62831==    by 0x404F10: run_argv (git.c:582)
==62831==    by 0x404F10: main (git.c:690)

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>