t9700: fix test for perl older than 5.14

t9700: fix test for perl older than 5.14

Commit d53c2c6 (mingw: fix t9700's assumption about
directory separators, 2016-01-27) uses perl's "/r" regex
modifier to do a non-destructive replacement on a string,
leaving the original unmodified and returning the result.

This feature was introduced in perl 5.14, but systems with
older perl are still common (e.g., CentOS 6.5 still has perl
5.10). Let's work around it by providing a helper function
that does the same thing using older syntax.

While we're at it, let's switch to using an alternate regex
separator, which is slightly more readable.

Reported-by: Christian Couder <christian.couder@gmail.com>
Helped-by: Dennis Kaarsemaker <dennis@kaarsemaker.net>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>