t3507: introduce pristine-detach helper

t3507: introduce pristine-detach helper

All the tests in t3507 (cherry-pick with conflicts) begin with the
same checkout + read-tree + clean incantation to ensure a predictable
starting point.  Factor out a function for that so the interesting
part of the tests is easier to read.

The "update-index --refresh" and "diff-index --exit-code HEAD" are not
necessary as the point of this testsuite is not about testing
"read-tree --reset".

Improved-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>