test: fix t5560 on FreeBSD

test: fix t5560 on FreeBSD

Since fd0a8c2e (first appearing in v1.7.0), the
t/t5560-http-backend-noserver.sh test has used a backslash escape
inside a ${} expansion in order to specify a literal '?' character.

Unfortunately the FreeBSD /bin/sh does not interpret this correctly.

In a POSIX compliant shell, the following:

x='one?two?three'
echo "${x#*\?}"

Would be expected to produce this:

two?three

When using the FreeBSD /bin/sh instead you get this:

one?two?three

In fact the FreeBSD /bin/sh treats the backslash as a literal
character to match so that this:

y='one\two\three'
echo "${y#*\?}"

Produces this unexpected value:

wo\three

In this case the backslash is not only treated literally, it also
fails to defeat the special meaning of the '?' character.

Instead, we can use the [...] construct to defeat the special meaning
of the '?' character and match it exactly in a way that works for the
FreeBSD /bin/sh as well as other POSIX /bin/sh implementations.

Changing the example like so:

x='one?two?three'
echo "${x#*[?]}"

Produces the expected output using the FreeBSD /bin/sh.

Therefore, change the use of \? to [?] in order to be compatible with
the FreeBSD /bin/sh which allows t/t5560-http-backend-noserver.sh to
pass on FreeBSD again.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>