ignore duplicated slashes in make_relative_path()

ignore duplicated slashes in make_relative_path()

The function takes two paths, an early part of abs is supposed to match
base; otherwise abs is not a path under base and the function returns the
full path of abs.  The caller can easily confuse the implementation by
giving duplicated and needless slashes in these path arguments.

Credit for test script, motivation and initial patch goes to Thomas Rast.
A follow-up fix (squashed) is by Hannes.

Signed-off-by: Junio C Hamano <gitster@pobox.com>