Ensure that we can `git_index_add` a slash-prefixed path, followed byre-adding the same path. The original entry should be replaced by thenew entry.

`has_dir_name` is used to check for directory/file collisions,and attempts to determine whether the index contains a file witha directory name that is a proper subset of the new index entrythat we're trying to add.To determine directory name, the function would walk the path stringbackwards to identify a `/`, stopping at the end of the string. However,the function assumed that the strings did not start with a `/`. If thepaths contain only a single `/` at the beginning of the string, then thefunction would continue the loop, erroneously, when they should havestopped at the first character.Correct the order of the tests to terminate properly.Credit to Michael Rodler (@f0rki) and Amazon AWS Security.