t1507: avoid "${parameter<op>'word'}" inside double-quotes

t1507: avoid "${parameter<op>'word'}" inside double-quotes

Kacper Kornet noticed that a $variable in "word" in the above construct is
not substituted by his pdksh.  Modern POSIX compliant shells (e.g. dash,
ksh, bash) all seem to interpret POSIX "2.6.2 Parameter Expansion" that
says "word shall be subjected to tilde expansion, parameter expansion,
command substitution, and arithmetic expansion" in ${parameter<op>word},
to mean that the word is expanded as if it appeared in dq pairs, so if the
word were "'$variable'" (sans dq) it would expand to a single quote, the
value of the $variable and then a single quote.

Johannes Sixt reports that the behavior of quoting at the right of :- when
the ${...:-...} expansion appears in double-quotes was debated recently at
length at the Austin group.  We can avoid this issue and future-proof the
test by a slight rewrite.

Helped-by: Johannes Sixt <j.sixt@viscovery.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>