os.path.relpath() needlessly calls abspath() when given two paths with matching anchors

[barneygale]

relpath() unconditionally calls abspath() to make both its arguments absolute before comparing them. This is necessary if the supplied arguments have different anchors, but it is not necessary when the paths' anchors match. We can save a call to getcwd() / _getfinalpathname() by skipping the abspath() call in this case. This should improve performance a little.

For example, neither of the following should require a call to abspath() internally:

relpath('foo/bar', 'baz/ding')
relpath('/etc/hosts', '/usr/local/bin')