GNU grep allows "\(A\|B\)" as alternation in BRE, but this is an
extension not understood by some other implementations of grep
(Michael Kebe reported an breakage on Solaris).
Rewrite the offending test to ERE and use egrep instead.
Noticed-by: Michael Kebe <michael.kebe@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>