This function was added in f103f95b

 in
the erroneous expectation that it would be used in the
reimplementation of longest_ancestor_length().  But it turned out to
be easier to use a function specialized for comparing path prefixes
(i.e., one that knows about slashes and root paths) than to prepare
the paths in such a way that a generic string prefix comparison
function can be used.  So delete string_list_longest_prefix() and its
documentation and test cases.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Jeff King <peff@peff.net>