Add tests to populate the working tree during clone and checkout using
the sequential and parallel modes, to confirm that they produce
identical results. Also test basic checkout mechanics, such as checking
for symlinks in the leading directories and the abidance to --force.

Note: some helper functions are added to a common lib file which is only
included by t2080 for now. But it will also be used by another
parallel-checkout test in a following patch.
Original-patch-by: NJeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: NJeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: NMatheus Tavares <matheus.bernardino@usp.br>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>