t2020: test variations that matter

t2020: test variations that matter

Because our test suite is not about validating the working of the
shell, it is pointless to test variations of how a literal string
'yes' is quoted when assigned to an environment variable.

Instead, test various ways to spell 'yes' (we use strcasecmp() so
uppercased and capitalized variant should work just like 'yes'
spelled in all lowercase) and make sure we take them as 'yes'.  That
is more relevant in testing Git.

Signed-off-by: Junio C Hamano <gitster@pobox.com>