t: use common $SQ variable

t: use common $SQ variable

In many test scripts, there are bespoke definitions of the single quote
that are some variation of this:

    SQ="'"

Define a common $SQ variable in test-lib.sh and replace all usages of
these bespoke variables with the common one.

This change was done by running `git grep =\"\'\" t/` and
`git grep =\\\\\'` and manually changing the resulting definitions and
corresponding usages.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>