t2080: fix cp invocation to copy symlinks instead of following them

t2080: fix cp invocation to copy symlinks instead of following them

t2080 makes a few copies of a test repository and later performs a
branch switch on each one of the copies to verify that parallel checkout
and sequential checkout produce the same results. However, the
repository is copied with `cp -R` which, on some systems, defaults to
following symlinks on the directory hierarchy and copying their target
files instead of copying the symlinks themselves. AIX is one example of
system where this happens. Because the symlinks are not preserved, the
copied repositories have paths that do not match what is in the index,
causing git to abort the checkout operation that we want to test. This
makes the test fail on these systems.

Fix this by copying the repository with the POSIX flag '-P', which
forces cp to copy the symlinks instead of following them. Note that we
already use this flag for other cp invocations in our test suite (see
t7001). With this change, t2080 now passes on AIX.

Reported-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Matheus Tavares <matheus.bernardino@usp.br>
Signed-off-by: Junio C Hamano <gitster@pobox.com>