t7810: avoid assumption about invalid regex syntax

A few of the tests want to check that "git grep -P -E" will
override -P with -E, and vice versa. To do so, we use a
regex with "\x{..}", which is valid in PCRE but not defined
by POSIX (for basic or extended regular expressions).

However, POSIX declares quite a lot of syntax, including
"\x", as "undefined". That leaves implementations free to
extend the standard if they choose. At least one, musl libc,
implements "\x" in the same way as PCRE.  Our tests check
that "-E" complains about "\x", which fails with musl.

We can fix this by finding some construct which behaves
reliably on both PCRE and POSIX, but differently in each
system.

One such construct is the use of backslash inside brackets.
In PCRE, "[\d]" interprets "\d" as it would outside the
brackets, matching a digit. Whereas in POSIX, the backslash
must be treated literally, and we match either it or a
literal "d".  Moreover, implementations are not free to
change this according to POSIX, so we should be able to rely
on it.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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