bisect.pl avoids perl-5.004 and earlier on case insensitive systems.

bisect.pl avoids perl-5.004 and earlier on case insensitive systems.

bisect.pl now probes to see if the checkout is on a case insensitive file
system (such as the default HFS+ on OS X), and if so, uses perl-5.005 as
the earliest stable release to test.

This should make bisect.pl "just work" on a typical OS X system, for the
vast majority of use cases.