#path#MakeRelative shouldn't strip trailing slash from {path} argument

[dbarnett]

The output of maktaba#path#MakeRelative doesn't have a trailing slash even if the {path} argument had one:
maktaba#path#MakeRelative('/foo', '/foo/bar/') → 'bar'

Since trailing slashes are significant in maktaba's path handling, explicitly signifying a directory path instead of a file path, it would be better to preserve any trailing slash from input to output.

[dbarnett]

Apparently this is just a manifestation of #137 since it's a call to maktaba#path#Split in the MakeRelative implementation that's losing the trailing slash.