Test t5560: Fix test when run with dash

Test t5560: Fix test when run with dash

A command invocation preceded by variable assignments, i.e.

	VAR1=VAL1 VAR2=VAL2 ... command args

are implemented by dash and ksh in such a way not to export these
variables, and keep the values after the command finishes, when the
command is a shell function.  POSIX.1 "2.9.5 Function Definition Command"
specifies this behaviour.

Many shells however treat this construct the same way as they are calling
external commands.  They export the variables during the duration of
command, and resets their values after command returns.

The test relied on the behaviour of the latter kind.

Reported-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Tarmigan Casebolt <tarmigan+git@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>