submodule foreach: correct path display in recursive submodules

submodule foreach: correct path display in recursive submodules

The `prefix` was put in front of the display path unconditionally.
This is wrong as any relative path computation would need to be at
the front, so include the prefix into the display path.

The new test replicates the previous test with the difference of executing
from a sub directory. By executing from a sub directory all we would
expect all displayed paths to be prefixed by '../'.

Prior to this patch the test would report
    Entering 'nested1/nested2/../nested3'
instead of the expected
    Entering '../nested1/nested2/nested3'

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